Hi Scott,
Wearing multiple pair of underwear would get tight at the waist as the elastic waist bands each add their tension.
Instead of that, have a look at what KINS offers at
http://www.kinsproducts.com20700 pull on cotton brief
20800 Terry pull on pants (they offer a double thick terry, too)
You'd have to wear waterproof pants over any of the above; see 10300 or 20300V
They also have an all in one; see 10900 pull on brief with a waterproof barrier.
They are a great company located in Canada and make high quality incon wear.
Good luck and let us know how you make out!
JoeK