Very Waterproof, however....once the sun gets to them that goes away very quickly. So may I suggest that if your going to store anything under them, for any length of time, that you double tarp them. The top tarp is the sacrificial tarp and the one under takes care of your items. At least that's what I do. Also the Silver Tarps are the heaviest 9mil so they cost a little bit more.
Tarps in general aren't waterproof since its just material coated in polyurethane. As soon as the sun eats the poly coating its just a big sifter. Better off buying a vinyl cover for a motorcycle or grill. If you don't want to spend much money buy the thickest plastic drop cloth you can find.
Agreed, even the best tarps won't keep moisture from ruining the seats pretty quick. I live in a really humid part of the country though. South Florida. My best results were making the tarp more tent like, rather than wrapping it in a tarp. It lets air flow but keeps the rain at bay.I find that keeping water out isn't as hard as letting the water out. Things covered with a tarp get condensation on them, then rust because the water is trapped under the tarp.
This! So true....and then there's the vermin that like being out of the rain and out of sight of predators while they feast on the wiring & seat foam.