Let's come up with an amphibian race!

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