In Imperator there is only one type of naval unit, the Trireme.
The main purpose of a fleet in Imperator is to ferry troops from one destination to another, and to stop others from doing the same in your territorial waters. Fleets can also blockade ports in wartime, decreasing commerce and potentially contributing to the warscore for their side of the conflict.
Building a Fleet
Ships can only be built in ports, and ports only exist in predefined locations around the map. Building ships also requires access to Wood in the Province that the City belongs to.