I agree with all the advice on going slow and staying relaxed. I was able to train myself to swim 50m underwater on one breath that way. To do that, start where you are (I think I started at 15m). Then swim that, but go "just one more" stroke before stopping. Soon that "one more" will be routine. Then go "two more" or "three more"--but only increase after you are relaxed at the new distance. Pretty soon 25m will be routine--just keep going like that and you'll build up to it.
Some mild hyperventilating (really deep breathing for 3-5 breaths, trying to empty lungs completely) pre-swim is useful, but don't overdo it--this lowers the CO2 levels and decreases the urge to breathe. If you OVERDO it, you can actually end up holding your breath until you pass out, which could easily prove fatal.
I think just about anyone can work up to 25m routinely if you build up slowly, and focus on staying relaxed in body and mind. I use the "frog kick-breast stroke arms-glide" technique mentioned earlier. Good luck!
Last edited by Tom Pamperin : 06-04-2015 at 05:59 PM.