I think I had just heard a TV commercial for CLEAR waterproof Gorilla tape yesterday, worth calling them.
But any duct or gaffer's tape should be waterproof, if you apply it to a dry surface, as opposed to trying to apply it to a wet one. Self-adhesive Velcro (preferably the "heavy duty" all plastic kind) might be another option, or 3M Command strips, that will still work when wet. Gear Aid Tenacious Tape (Amazon, etc. or camping suppliers) works on raingear and tent flies, thinner than duct tape but sticks well too. Available in thin flat "rolls" that will pack down well, too.