As a kid, I fondly remember dropping objects to the bottom on the deep end and swimming the 10 feet to the bottom to get them. It developed good breathing, muscles and good planning skills. Six feet just wouldn't have done it. Children need this sort of activity, whether it's in swimming, biking, dodgeball or whatever. All this silly take-away-the-risk emphasis is breeding sissies.
When, again, is diving not an option?