In another game, in another life, I have been a Priest of Fish (the god
called Fish, he's rather literally named), and I spent far more time than necessary just walking around on the seabed, once I could perform the Breathe Underwater ritual sufficiently well. Occasionally rescuing players who accidentally came to me by magical means (though, admittedly, if I hadn't been wandering around underwater, they'd have probably acidentally arrived at my/another's location
without the need to be so rescued). Certainly back then, the (underwater) terrain was a little sparse of shipwrecks (there were some things, and a branch of the temple to Fish actually at the bottom of a river, a nice little getaway) but there was at least some submarine fauna to occasionally wave a bastard sword at (and maybe occasionally some to
not do that, with too much haste).
I certainly don't see it as a bad idea to add to the explorable terrain. But it might be a triple-edged sword if it has to all be done on a single breath, between resurfacings. At least until you can construct a 'diving refuge', either just a simple large-scale 'bell' or something
more complicated that probably has to wait for the "movable forts/buildable ships" update to implement properly...
...and, of course, mermaid farming will make a comeback with a vengeance! (Or land-dweller farming, if actually playing a mer-race, such as Sea Devils..?)