If you know how to swim then it's quite hard to drown unless the water's so cold your muscles bail out on you. (Former youth competition swimmer speaking. ) If you start getting incredibly tired you can always just float at the surface, waving your arms slightly to stay boyant. And keep plenty air in the lungs. Of course, doing that won't win you any trophies, lol. Isn't the swimming bit the first leg of the triathlon though? Then you should have no problem making it out of the river alive.