Ok. That's why I wrote "provided your ball doesn't collapse under the water pressure". By "collapse" I meant to include "compress", but didn't state that clearly. A reversed example of this kind of thing is air bubbles rising in water. They start of small and rise slowly, but as they go up the water pressure decreases and the bubbles expand. As they expand, the buoyant force increases and they rise more rapidly. I don't know if you notice that kind of thing when you're diving.