Aqua-tite works well also.

303!! Amazing stuff, makes the water look like mercury as it beads off of it.

I just had my 36 Spectre cover washed by G&K a commercial uniform company here in VA for $56.70. I ordered 2 gal of 303 from Amazon $173.97 and will be treating it tomorrow in my basement with sheet plastic laid down first. The boat just came out of the paint shop and I could not put a dirty cover on it.