probably before I learned to walk.. But, as we've discussed here, before, you really don't have to be a good swimmer, or even know how to swim, to dive.. I wouldn't advise anyone to learn to dive if they didn't know how to swim, but it is possible..
..and I think your dad's plan would have worked.. its actually quite brilliant.. Would be harder to do in the ocean since its almost impossible to sink in the ocean without weights..
Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..