"ABZÛ is an epic descent into the depths of the sea where players explore beautifully rendered ocean environments with fluid swimming controls. In ABZÛ, the user embarks on a nonviolent, 3rd person adventure with no fail states so users can’t die. They can only explore and discover new things about the underwater world as they progress along the emotional spectrum of the game."
I've played a lot of Subnautica - dozens and dozens of hours - and I think the thing that seems the most different in ABZÛ is the speed and movement of the diver. There are points where they're positively zooming along, even doing dolphin-like flips out of the water.
There's actually a cool dev blog on the developer, Giant Squid's, website which looks at how they made the swimming work - there are really interesting touches like the camera predicting where the diver will be facing after she drifts a little at the end of the movement input, or how loop-the-loops work.
I wonder if the game itself will end up somewhere between Subnautica and Endless Ocean? I guess I will find out on 2 August!