Let's come up with an amphibian race!

But then there really is an imbalance. Maybe we should still make the Fortress units able to sail on ships, just faster. Or give the land races the ability to sail on ships better.

We need to think about it some more. It just doesn't seem right to require a ship to move across the surface of the water for those who live underwater. Perhaps, in that case, we need to think about other options and connect the seas.
P.S. I'll be away for an hour.