I could always buy this:
exhaust wrap spray - Google Product Search and spray it all over the wrap after im done wrapping it, it says it will protect the wrap from spills.
Though I dont think it will be a problem if the corrugated plastic will enclose the whole underside of my car, and im planning on that, so I dont see how the water can get to the exhaust unless i literally went into a puddle slow enough for the water to soak through.
Is this safe to do though? To enclose the whole underside after i heatwrap it?