if they made something like that I would consider buying it. I have thought about carrying one of those seat cushions for that purpose as well. I have gone over many (well... enough) times and never gone turtle but I have been afraid of doing it.
I didn't even have a problem when I had to swim the thing to shore (don't worry Pete, this was all my stupidity- at the time- and the first time I had the boat out).

Hey, while we're at it- how do you make sure your mast is water-tight? If it has a leak up top but sealed on bottom then won't you just accumulate water and create a bigger problem?