Like some of the other reviews, when you first open up the packaging you think, "Well that's not going to work, is it?"
I use it to put my car keys with remote door opener in there whilst I go surfing. It takes a few uses to build up trust but it really works.
Here's my little tip :-
Turn up at chosen surf spot and wear this thing round your neck with the keys in it while you faff about. I do this so potential thieves can see I have one and I will be taking the keys with me. Get suited up, and walk down to the waves, still with this around my neck. I usually dump a towel and bottle of water on the beach somewhere. Then I walk down to the water and take the Aquapac from around my neck and tuck it inside the base of the wetsuit zip at the small of my back - in view of other potential thieves. Zip up your suit and go surf. At any point you're feeling paranoid you've lost it you can just feel it through you suit. I really like the idea that if someone's been watching they can see I have taken my keys with me.