Yeah, the only bigger (non-MMO) open-world games are Just Cause 1/2, FUEL, and maybe the little-known Boiling Point/White Gold AFAIK.
And there's all that area underwater to explore too, coves, tunnels, trenches, sunken ships, hidden entrances...
Just beware of the sharks, as you only have the knife usable whilst scuba-diving. I tried punching one on the nose... It didn't work out
quite so well.