Let's come up with an amphibian race!

Yes, exactly!
We need to solve problems...
For example, ban submarines from leveling up the Guild of Mages above level 3...
And remove the ability to drown/dry out, and instead impose penalties. And equalize the levels for entry/exit...
There's a lot of work to do, but this is just the beginning

Added after 56 seconds
And give submarines a VERY small bonus for powerful monsters.

No problem. And regarding the descent/ascent, we can make it like a shipyard building, where mages will sell a magical sphere, with water and air inside. And they can travel in it.