Let's come up with an amphibian race!

If the seas aren't connected, then the optimal solution is to have a different chemical composition for the surface and underground seas. For the Gar'goyles, the surface sea is like acid. And the underground sea would be normal water for them.

No. That would be too much. We've already discussed a lot. An underwater hero moves on the surface of the surface sea without a ship, but on land, he moves with a penalty equal to, say, 25% of the normal movement speed of a land hero. Movement can be increased by acquiring a horse at the stable (where else?) or at the exit from underwater.