I love swimming in the ocean especially deep sea diving or underwater diving.
Interesting, particularly because humans are not exactly built for the environmental conditions of diving! 
Yeah. The professional free divers can hold their breaths for 5 minutes and longer I think.
Hmmm, I don't know a lot of free divers, but I think that more of the conern with diving is the pressure of the water.
Free diving is a professional sport where divers go down as deep as they can to win. But not every one can do this because of the water pressure and conditions that effect the body as you have stated.
That's right! Most of the divers who blackout are actually experienced and strong, in fit condition.