Let's come up with an amphibian race!

The diving school shouldn't be in this city, but on the surface. And in this city, a special building could be something like water gates — movement between cities of this race.

I was talking about the surface. Well, yeah... like Inferno had in own three.

Added after 3 minutes
And as for the school, I think there should also be three levels of training: novice — 1-2 turns underwater, master — 3-5, expert — as long as you want. The duration can be discussed, but the main thing is that at the very last level, you could stay underwater indefinitely. Or maybe a skill like "Basics of Diving" (without a school) could be introduced with subsequent development?

Added after 1 minute
Probably not worth it — the skill tree would have to be redone.