I'd recommend plastic sheet or even tyvek for the groundcloth over silnylon for a couple of reasons. For one, the waterproofing on silnylon isn't that durable. It's fine for keeping rain off, but lay it on top of standing water any pressure on it from kneeling or sitting will cause it to soak through. Plus your groundcloth will get beat up, no reason to spend $50+ for a piece of silnylon when a sheet of free plastic will do.