I seem to remember that one of the issues the "Mythbusters" had with ping-pong balls, was all the dead space between the balls. They found the balls to be quite inefficient 'lifting devices', hence the massive amounts of balls they used.

When they tested the myth about beeing dragged down with a sinking ship, they did not use ping-pong balls to raise the "Mythanic" between tests. (The "Mythanic" was a large and heavy metal boat, vs. the small sailboat prepped and raised with ping-pong balls)