At least it doesn't look like the Hindenberg following you all around the lake overhead. Sorry, I think the Big Bob's look funny.
How about a big rubber ducky? or a little model of a pirate in a crow's nest carved from Styrofoam?
Back to reality: I have seen some sails, somewhere, on some boat, that have a thin panel of lightweight, flexible closed cell foam sewn into a pocket on the very top panel of the sail for floatation. It actually IS the same size and shape of the top panel - so you have low-windage, integrated floatation rather than an add-on. The foam is like the stuff that comes in sheets that pool installers use to line the bottom of pools- (brand name "Happy Bottom Pool liners) it is 1/4 or 3/16 thick. There is always a bunch left after installations that your local pool-guy probably throws away. You could probably get a sail maker or canvas shop put together something for you.
Thinking outside the box, and off the top of the mast!