I have used ScotchGuard Fabric protector spray with success, but it's expensive and only lasts for one season. I paid around $12 per spraycan, and it took 4 cans. Still, with a quote of $800 for new bimini canvas, I'm going to get as much life out of this one as I can!
The Neverwet is an interesting idea, but be warned that it is not clear. It ends up as a frosty translucent white.