Abzu? Dave the diver, maybe? Dredge?

Or, this might be a might more out of left field or maybe completely stupid and not at all what you had in mind, but what about coral island? You don’t get to experience the water surface except when fishing or doing other stuff on the shore. But you get to do stuff on the bottom and there’s mermaids and stuff.

Stardew valley with a tropical island vibe. So definitely fun and immersive. Definitely more adjacent than something like sea of thieves or black flag or subnautica, but I feel like it fits better than just cause 3, imo