I have several Seal Line bags (like 8 or 9 of them in different sizes from size 5 to size 30) that look to be the same thing as what is pictured in that link. I use them for canoeing and camping, they are very strong and just short of being put under water and squeezed to death seem to be waterproof.

They also work well for washing clothes, I put some water, laundry soap and the dirty clothes in them, seal them up and shake and flip them back & forth for a few min. then rinse twice the same way. Makes for a good way to clean clothes while out in the wild.