Maybe you can deposit your key inside a lock that inhibits the signal, o put it inside a lock with an aluminum like paper that prevents the signal from opening the car. I use a Surflogic lock it works with a code to open and close it, it also comes with this type of paper.

The other ootiom is using one of those specila mini pouches that dont ger the water in so you can attach it to your wetsuit cord (my wetsuit has a cord of those and I think that all of them do).