Depends on the situation, which is pretty much the case for any game. You have a strategy you want to carry out however the current game position says you should do something else. I always produce a good bit of subs because they light things up and for cheap. Not to mention they can take out a whole fleet of big ships if they aren't protected properly. I use them like light bulbs all around the waters surrounding my territory, kinda like sonar buoys.

That being said though on the big maps with plenty of water and you can't win the game unless you cross it and take over more territory then yes you would do well to produce a decent fleet to escort your troops across the waters. If you get lucky enough to be in a game long enough with active players you will see that a kick ass Navy is key to winning the game. If they can't get their troops to your lands they can't fight you on them. If they can't stop you on the water then they loose simple as that.