I was out on the ocean in a boat, on top of the water, standing completely still and closed the game.
When I logged in again, I was under water, in my boat, and was going slowly up to the surface (as you normally would do when being in a boat). However, my "breath meter" was going down (Also normal) and when it reached 0 I thought I would start taking some damage. This was not the case, I was still going slowly up and not taking any game from the lack of oxygen.
A visual bug maybe?

I have later tried doing this again a couple of times but couldn't replicate the bug.
My PC was running slow when I closed the game... Not very slow, but slower than usually....