Finally, a simple guide.
Naval units have a limited number of uses, but in those fields, nothing else comes close. Sooner or later, everyone needs a Navy. No exceptions.
You know how this works: I will list every single naval vessel and how they should be used, as well as when is a good time to use them. Let's meet out boats:
We have:*
-Destroyers
-Cruisers
-Submarines
-Battleships
-Aircraft Carriers
*I'm just going to put this here now: I'm not counting naval transport. They aren't a true navy unit, but rather a segway to getting your units overseas.
---DESTROYERS---
What they can do:
-Intercept incoming transports. Destroyers are wicked fast, capable of hitting targets quickly and easily. It takes a naval transport a few hours to disembark onto land, so hit them before they can!
-Sub hunters! Destroyers are the only - and I do mean only - naval unit capable of finding subs. This is not a joke: you NEED destroyers in every single stack. Send those subs packing!
-Scout islands, especially in the Pacific. Destroyers are fast, making them perfect for scouting those single-island provinces to look for fortifications.
What they can technically do, but shouldn't:
-Ward off aircraft. Slightly higher AA capabilities than a battleship, and much higher than a transport. It kinda works, early game. That being said, don't do this if you're expecting aircraft. Use a cruiser. It's much more effective.
-Harass ground units. As of 1.5, the destroyer can finally attack ground units... kind of. The truth is it's very ineffective, outputting a measly 0.5 damage at level 1. It also has a really short range, making its targets limited. Can it be done? Technically, yes, but I would limit this to early game only, and not against artillery.
What they can't do:
-Take territory. This is a given.
-Keep a distance. Destroyers have a very short range, and need to get within firing distance of every other ship in order to work.
---CRUISERS---
What they can do:
-Ward off naval bombers. This is an important one! If you want to defend your ships from the perils of naval bombing, cruisers are a must. This is non-negotiable!! Yes, technically you can stack several destroyers or battleships to make up the difference, but this is a waste of resources: just build a cruiser!
-Hit much harder than destroyers. Cruisers have way, way higher attack power against virtually every unit.
-Hit further away. Has about double the range of a destroyer.
What they can technically do, but shouldn't:
-Once again, hit ground targets. Now, this is much more effective with a cruiser, but given the low attack values of a cruiser against ground units, it's kinda debatable. Personally, I would do this, but I'm biased. It definitely depends on the situation, and don't expect a ton of damage output, but it's 100% better than nothing.
-Defend your coastline against amphibious assault. In my personal opinion, cruisers are really bad at this. It's nothing personal, but cruisers are a lot slower than destroyers, and while they have longer range, you still have to get pretty close to hit the targets. It's just not the best option.
What they can't do:
-Defend against subs. They just can't. At later levels, they gain a little defensive power, but not enough to be without destroyers.
-Go up against a battleship. They're almost completely outclassed in every single regard, offensively.
---SUBMARINES---
What they can do:
-Scout. They can find anything. Why? Because they're stealthy. At all times, really.
-Sink battleships and transports with ease. Not because it's inherently powerful, not really, but rather because those units are defenseless against subs.
What they can technically do, but shouldn't:
-Sink cruisers, albeit with a bit more difficulty. Cruisers are not defenseless. However, more often than not your attack power still outclasses their defense, so you still can. However, beware of higher level cruisers! They present a real challenge.
What they can't do:
-Face off against destroyers. Seriously, you don't have a chance.
-Face off against other subs. This... surprised me. The reason why is because you technically can't find them. You're a stealth unit, and so are they, so you just have to hope you run into them. And even then... your attack values kinda suck. Just use destroyers.
-Defend themselves against naval bombers. Actually, they can hardly defend themselves against aircraft at all. Just don't do that.
---BATTLESHIPS---
What they can do:
-Strike the shores. Finally, a naval unit that can do some damage! The battleship is the only naval unit capable of doing real harm against ground targets. It has limited range, but those coastal provinces won't know what hit 'em!
-Wreck havoc on other naval targets. The battleship has the highest attack values against navy and surface targets out of every naval vessel. Paired with the longest range, it makes for a deadly combo.
-Annihilate transports. Don't wanna be fast? Just use sheer brute force. Where the destroyer is good for chasing down transports, this unit is good for making sure they don't reach the shore to begin with.
What they can technically do, but shouldn't:
-Nothing.
What they can't do:
-Defend against Aircraft, specifically naval bombers. Remember how much I stressed cruisers?
-Fight off submarines. These bad boys have virtually no defensive power against these little sneaks!
-Be mass produced. Battleships are expensive as hell, and they aren't fast. They take a long time to build.
---AIRCRAFT CARRIERS---
What they can do:
-Function as a mobile airstrip. You can fit a limited number of aircraft on an aircraft carrier, and you can only fit certain kinds, so be sure to pick aircraft according to what you need!*
*IN THE EARLY GAME, I would recommend sending Aircraft Carriers in pairs, to maximize carrying capacity without overstacking. However, starting at level 3, don't keep 2 aircraft carriers in the same stack anymore.
What they can technically do, but shouldn't:
-Function as an Anti-Air battery. THESE DO NOT REPLACE CRUISERS. However, they do have a decent anti-aircraft damage amount.
What they can't do:
-Anything else. Air Carriers can't attack ground units, and can't ward off naval vessels or submarines. They're defenseless. They literally only function as an airstrip, so use them as such.
Naval units have a limited number of uses, but in those fields, nothing else comes close. Sooner or later, everyone needs a Navy. No exceptions.
You know how this works: I will list every single naval vessel and how they should be used, as well as when is a good time to use them. Let's meet out boats:
We have:*
-Destroyers
-Cruisers
-Submarines
-Battleships
-Aircraft Carriers
*I'm just going to put this here now: I'm not counting naval transport. They aren't a true navy unit, but rather a segway to getting your units overseas.
---DESTROYERS---
What they can do:
-Intercept incoming transports. Destroyers are wicked fast, capable of hitting targets quickly and easily. It takes a naval transport a few hours to disembark onto land, so hit them before they can!
-Sub hunters! Destroyers are the only - and I do mean only - naval unit capable of finding subs. This is not a joke: you NEED destroyers in every single stack. Send those subs packing!
-Scout islands, especially in the Pacific. Destroyers are fast, making them perfect for scouting those single-island provinces to look for fortifications.
What they can technically do, but shouldn't:
-Ward off aircraft. Slightly higher AA capabilities than a battleship, and much higher than a transport. It kinda works, early game. That being said, don't do this if you're expecting aircraft. Use a cruiser. It's much more effective.
-Harass ground units. As of 1.5, the destroyer can finally attack ground units... kind of. The truth is it's very ineffective, outputting a measly 0.5 damage at level 1. It also has a really short range, making its targets limited. Can it be done? Technically, yes, but I would limit this to early game only, and not against artillery.
What they can't do:
-Take territory. This is a given.
-Keep a distance. Destroyers have a very short range, and need to get within firing distance of every other ship in order to work.
---CRUISERS---
What they can do:
-Ward off naval bombers. This is an important one! If you want to defend your ships from the perils of naval bombing, cruisers are a must. This is non-negotiable!! Yes, technically you can stack several destroyers or battleships to make up the difference, but this is a waste of resources: just build a cruiser!
-Hit much harder than destroyers. Cruisers have way, way higher attack power against virtually every unit.
-Hit further away. Has about double the range of a destroyer.
What they can technically do, but shouldn't:
-Once again, hit ground targets. Now, this is much more effective with a cruiser, but given the low attack values of a cruiser against ground units, it's kinda debatable. Personally, I would do this, but I'm biased. It definitely depends on the situation, and don't expect a ton of damage output, but it's 100% better than nothing.
-Defend your coastline against amphibious assault. In my personal opinion, cruisers are really bad at this. It's nothing personal, but cruisers are a lot slower than destroyers, and while they have longer range, you still have to get pretty close to hit the targets. It's just not the best option.
What they can't do:
-Defend against subs. They just can't. At later levels, they gain a little defensive power, but not enough to be without destroyers.
-Go up against a battleship. They're almost completely outclassed in every single regard, offensively.
---SUBMARINES---
What they can do:
-Scout. They can find anything. Why? Because they're stealthy. At all times, really.
-Sink battleships and transports with ease. Not because it's inherently powerful, not really, but rather because those units are defenseless against subs.
What they can technically do, but shouldn't:
-Sink cruisers, albeit with a bit more difficulty. Cruisers are not defenseless. However, more often than not your attack power still outclasses their defense, so you still can. However, beware of higher level cruisers! They present a real challenge.
What they can't do:
-Face off against destroyers. Seriously, you don't have a chance.
-Face off against other subs. This... surprised me. The reason why is because you technically can't find them. You're a stealth unit, and so are they, so you just have to hope you run into them. And even then... your attack values kinda suck. Just use destroyers.
-Defend themselves against naval bombers. Actually, they can hardly defend themselves against aircraft at all. Just don't do that.
---BATTLESHIPS---
What they can do:
-Strike the shores. Finally, a naval unit that can do some damage! The battleship is the only naval unit capable of doing real harm against ground targets. It has limited range, but those coastal provinces won't know what hit 'em!
-Wreck havoc on other naval targets. The battleship has the highest attack values against navy and surface targets out of every naval vessel. Paired with the longest range, it makes for a deadly combo.
-Annihilate transports. Don't wanna be fast? Just use sheer brute force. Where the destroyer is good for chasing down transports, this unit is good for making sure they don't reach the shore to begin with.
What they can technically do, but shouldn't:
-Nothing.
What they can't do:
-Defend against Aircraft, specifically naval bombers. Remember how much I stressed cruisers?
-Fight off submarines. These bad boys have virtually no defensive power against these little sneaks!
-Be mass produced. Battleships are expensive as hell, and they aren't fast. They take a long time to build.
---AIRCRAFT CARRIERS---
What they can do:
-Function as a mobile airstrip. You can fit a limited number of aircraft on an aircraft carrier, and you can only fit certain kinds, so be sure to pick aircraft according to what you need!*
*IN THE EARLY GAME, I would recommend sending Aircraft Carriers in pairs, to maximize carrying capacity without overstacking. However, starting at level 3, don't keep 2 aircraft carriers in the same stack anymore.
What they can technically do, but shouldn't:
-Function as an Anti-Air battery. THESE DO NOT REPLACE CRUISERS. However, they do have a decent anti-aircraft damage amount.
What they can't do:
-Anything else. Air Carriers can't attack ground units, and can't ward off naval vessels or submarines. They're defenseless. They literally only function as an airstrip, so use them as such.