Let's come up with an amphibian race!

If we're talking about a land-based sea, it can be done like this. The Fortress faction will have the ability to create a ship right on the shore (the ship will serve as a symbol of a unified army, preventing them from scattering in different directions). Other factions will be able to buy or summon a ship, while the Fortress faction will simply create one. This will be one of their unique abilities.