Yep, I hear you. An underwater setting would be a dream come true for me. I even started to homebrew one for 5e, but it's really difficult to pull off. The biggest hurdle IMO is the Atlantis is Boring trope. Solving that problem is not as easy as it may seem. The setting itself might be cool, but then you have to write up an interesting adventure for your players, with plot hooks that make sense and that will keep them engaged, and for me that's incredibly difficult to do with an underwater setting, at least beyond the first two or three sessions, after the novelty of the setting has worn off.
DnD oceans aren't "just fish and seaweed" as that link implies, that they are full of actual sentient NPCs, with towns and potential companions and problems to solve. Aquatic elves, sirines, sahuagin, aboleth, storm giants and their underwater castles, getting to see the Citadel of Ten Thousand Pearls, an entirely underwater city on the Elemental Plane. Many possibilities.
As an aside, I make almost all of my PnP characters followers of Trishina.