A lot of hentai sites (sorta understandably) don't have a "scuba" or "diving gear" tag and even if they do most artists don't bother adding specific tags like that so there is so much good shit out there that we don't know about because it's only tag that hints about it's content is "underwater" so if your really desperate for stuff you should search for hentai with the "underwater" tag on hentai sites and you'll sometimes find some scuba stuff.
Sauce:
https://nhentai.net/g/87463/ 