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.
established practitioners of breath-hold diving, are fit, strong swimmers and have not experienced problems before. Blackout may also be referred to as a syncope or fainting.
Divers and swimmers who black out or grey out underwater during a dive will usually drown unless rescued and resuscitated within a short time. Freediving blackout has a high fatality rate, and mostly involves males younger than 40 years, but is generally avoidable. Risk cannot be quantified, but is clearly increased by any level of hyperventilation.
Freediving blackout can
The ancient technique of free-diving requires breath-holding, and world-class free-divers can hold their breath underwater up to depths of 214 metres and for more than four minutes. Apneists, in this context, are people who can hold their breath for a long time.
Hyperventilation.
Voluntary hyperventilation before beginning voluntary apnea is commonly believed to allow the person involved to safely hold their breath for a longer period. In reality, it will give the impression that one does not need to breathe, while