Underwater rock carrying is something I saw during BJ Pen training for his coming up UFC fight. I found out this is an old traditional Hawaiian Warrior Training method. Surfers use this method to improve their overall conditioning. Some expert's work I read on this topic believe that this is form of hypoxic training that leads to increases in lung capacity.
This type of training should be done by elite athletes only and should be done under supervision due to the possible dangers associated with it.