Fighting on water is very different than fighting on land. For one thing, there is usually less space to maneuver and the units are much more limited. The downside to naval units is that they are not amphibious, and once you have conquered the seas you will have little opposition in those areas of the maps. However, if there are any resources along the coast, your naval units can help defend them.
there are several types of warships available in Age of Empires II
1. Fishing ship : used to gather food by fishing.
2. Trade cog : used to trade with other allies or players.
3. Transport ship : used to transport units to one place to another
4. Galley : used to defend your base and attack the enemies base it can be upgraded to a War Galley once reached the Castle Age and it can be upgraded into a Galleon in the Imperial Age.
5. Demolition ship : used as a suicide unit it is upgraded into a Heavy Demolition ship in the Imperial Age by some Civilizations.
6. Fire Ship : used as a support unit when defending and attacking enemies
7. Cannon Galleon : can be researched at the Dock when Chemistry has been Researched used as a Siege unit