Paint below the waterline

If they use swimming pool paint to keep water in a pool, why can't you use the same paint below the water line to keep water out of your hull?
Why can't you use something ruberized like Sno Roof or Rhino Liner?

 

If your pool leaks, the water runs into the sand. The worst that can happen is that the pool floats a bit and cracks. The converse is that if your goo leaks, the boat doesn't float.