I have had the good fortune, or cautiousness to never get very close to blacking out in the ocean.
In my teenage years I have done it twice in a swimming pool (unsupervised and with friends). We used to do underwater swimming contests.
I have the good fortune of seeing lots of stars (black & white static at the edge of vision) before blacking out for at least a couple of seconds. And watching my vision tunnel to blackness. But lots of people just go black! and suddenly!
Doing 200 yard (Amercian swimming pools are 25yards [23m] long) underwater swim races I have blacked out and floated up.
Lots of factors change when this happens all the things that make it harder to hold your breath. Activity, breathing, neverousness, diet, getting shocked by somehting.
