Wetbag = a somewhat waterproof bag for carrying used cloth diapers, mama cloths, wet swimsuits and more.
The best part about a wetbag is you can launder it along with any of the above. That's tres convenient in my book.
The inner lining is commonly made of PUL (a tightly woven poly with a plastic-y backer. It's found in many modern cloth diapers). You could also use other types of nylon - think windbreakers.
Cut four identical squares of whatever size you'd like to make your bag. Two from cotton fabric, two from PUL or nylon.
Layer the two fabrics with their wrong sides together. One set of these is the front of the bag and one set is the back.
Lay your zipper upside down over the top of the outer fabric (your PUL is hiding underneath). Stitch the zipper in place across its top edge.
Do the same thing with your other wetbag pieces - lay the zipper in the same position and stitch across the top edge. Finish the inside seam. (View from the inside)
(View from the outside). Open your zipper halfway.
With right sides together, sew the sides and bottom of your bag closed. Zigzag the raw edges if preferred.
The half-open zipper will allow you to turn the bag right side out.
I throw my wetbags in the washer and dryer on a regular basis. The PUL holds up extremely well!
Looking for PUL?
I like http://www.wazoodle.com/ (a Canadian online shop)