With a strong head wind blowing it could be quite a lot - have you tried carrying a big inflatable racing buoy on your boat in a wind? The tubes on a RIB increases it's surface area quite a bit.
It's not just my way of thinking - the late great Shaun White said something very similar in one of his articles.
Also watch Mythbusters - the one where someone survies a drop by sitting ontop of an inflatable raft.
Obviously hull design has a lot to do with it - a deep V will fall back to earth much quicker than a flat bottomed boat - look at the way a CAT flies up if the nose gets too high.