Let's come up with an amphibian race!

I suggest the following: For land units to descend underwater, they need to obtain a water sphere (or gain the Diver skill). While underwater, they have a penalty, but an expert Diver eliminates it. Amphibious units can emerge onto land by obtaining an earth sphere (or gaining the Land Walker skill). While on land, they have a penalty, but an expert Land Walker eliminates it. All units travel across the surface sea on a ship (its chemical composition differs from the underground sea, so amphibious units cannot travel on it without a ship).