Hi Forodin (aka Shane)

Welcome to the Guild!

You need two things: A harbour (building - does not require fort, four turns and 1500 florins IIRC)
A ship in the seazone where the port is. You can tell by the physical placement of the port in the campaign map. But a safe guess is the seazone between Corsica and mainland Italy.
The ship must be, in the beginning of the turn, without enemies - that is, if one of the factions you are at war with moved a ship into the zone the last turn, you'll have to wait for ship combat to be resolved at the end of turn. Once the seazone has been cleared for enemy activity, you can move your army back to your own provinces, or lauch an assult somewhere else.



/KotR