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Games where you actively swim the entire time? That's pretty much boiled down to Sea of Thieves, Subnautica, and ABZU. There's also an impressively fleshed out underwater environment in GTAV, though that's certainly not going to be 90% of your time in that game unless you suck at using the sub and are trying to get all of the collectibles.
What Zombody2 said. Plus Abzu is an absolutely beautiful game. I first played it on the PS4 and am thinking of playing it again on Xbox1. It is a tad short but it is an easy 100% gamer score. I'd keep an eye out for when it goes on sale.