Let's come up with an amphibian race!

Remove it, but not for everyone, only for those who have this sphere. No sphere - look for a ship.

No... It won't work. Since the entrances to the underwater level will not be everywhere, those who have such an entrance on their territory (and that's where you can acquire such a sphere) will have an advantage in moving across the land sea compared to those who don't have such an entrance.