BryanW gave admonitions that I started to give. DON'T make this competitive. Swimmers at the competitive level USED to practice this in the dark ages. Sort of like don't drink water during football practice. They were so competitive, some were found at the bottom of the pool, blacked out, but refusing to surface for air. A couple of famous incidents occured at Stanford where team members were purported to be swimming over a hundred of yards under water. Fortunate that death or permanent brain injury did not result. (Brain damage? Unlikely! Hell, they were swimmers!)
Tyson