You could use dyneema (they sell some here but you have to fight your way through their strange system to get a quote). It is strong, floats and has good UV resistance. It is really wonderful stuff and is being used commercially to replace wire for towing, trawling and mooring oil rigs. It's probably just about ideal as a dink painter. But, you could buy a whole lot of poly-pro for the same money. Keep in mind, not all poly-pro is the same. There are some poly pro lines that have a decent hand and hold knots adequately. The best poly-pro (like the stuff they call "Silver Line" down under) is actually pretty decent rope.
--Tom.