I just wanted to clear up what a winglet does. It simply makes the wing more efficient. It does this by forcing the air to flow over the end of the wing as it does in the middle of the wing, straight back. Without the winglets (and the same hold true for our sails) the air near the tip spills off toward the end instead of straight back. On our cats we lose at the top and the bottom.

I think a float that was airfoil shaped could work on square top sails. I don't think it would need to provide lift. Thats the last place you want to create drag.