Hm, the only underwater ones I can think of are Subnautica and Abzu, both of which are still in my library (although I wouldn't mind if Abzu went away, as it was basically just an underwater walking sim). I couldn't find anything on the list quickly, despite a couple ones sounding like underwater ones, like Stranded Deep and Submerged: Hidden Depths, which didn't look like they actually were underwater games.

https://gamerjournalist.com/epic-games-free-games-list/