I have the Bagallini (LG) wallet bag in black, it is my go to travel bag. I have never submersed it but it has fared well in Orlando's torrential downpours as well as all water rides at WDW and other assorted amusement parks.
My one suggestion is to get it ahead of time and to actually wear it the way you plan to. I find that even the wallet bag which is very small can irritate my shoulders and back when I haven't used it in awhile.
I bought mine at a local luggage store in our area several years ago. It was in the $40 range but it has more than paid for itself over the years.