Ships can be moved anywhere ON SEA, you can move them to the shores regardless if there is a harbor or not, but they can never enter a city, and will not fight an enemy attacking the city unless in the range of their fire.

Well, not exactly.

I have seen very weird scenarios in which enemy ground units (e.g., armor or infantry) and naval units (e.g., destroyers) make physical contact at the water' edge and are locked into combat even though their respective unit spec sheets indicate that they have no offensive or defensive power against the other unit type(s). I saw this occur at least twice last summer, one involving my own naval units, and in the other case ground units of one of my allies. The deadlocked situation involving my naval units was resolved when I killed the enemy ground units with attacks by my tactical bomber wing. The deadlock involving my ally's ground units and enemy naval units had to be resolved by a game operator, who separated them.

This problem may (or may not) have been resolved by subsequent software fixes. Until I hear otherwise, I have made sure that none of my naval units make physical contact with enemy ground units (and vice versa).

Well, that's weird, from what I understood it's a rare bug rather than an actual game mechanic, but it's a good advice to keep in mind, thanks.

in fact it's a game mechanic as all the units have a little range to engage in battle, both, sea and land. so if the city is so close to the beach then yes, a ship can engage in battle with the garrison of that city of the units in battle there

same happens when you are in different lines(paths) and if they are very close they can engage in battle

Yeah, enjoy that endless destroyers vs. infantry/armor battle where they are locked together for eternity, but can inflict no damage on each other.

I suppose you could use it as a tactic to lock the enemy units in place until you can bring other effective units into play, but you risk the same.