My swag is a 100% 15oz canvas standard swag and I just put it straight onto the ground. Wet, dry, mud, dirt, rain etc.
It has a thin high density foam mattress for comfort and insulation from the ground.
The only moisture I've had in the swag is from my breath when I put the flap over my head in bad weather.
Just remember to drape it over something during the day to dry out. I assume moisture would soak through eventually if you leave it on the ground for days on end.