When Arnold and Summer are on the raft, Helga spits a small dart at it. After that, the couple are thrown upward, then into the water. Unless there is a sudden, split-second change of gravity, this is pretty much impossible.
I seem to agree with you. If the floatie had enough pressure, it probably could have done it, but, no one would fill their floatie up with that much air. If you don't catch my drift, IT'S ONLY A CARTOON!
Let me put it this way. Its a cartoon!!! The idea is for the viewer to laugh. It's like saying, "How could Wile E. Coyote survive an explosion, fall from a cliff, or whatever?".