I use a small waterproof pouch on a lanyard. Works perfect.
For most locations, I lock my wallet in the car. In some places (like night diving in maui) is is not a good idea. Some locals will wait for you to drop down and then break into your car. When I am in places like this, I strip the car of anything worth stealing and leave it open and unlocked.
FWIW: This chipped keys seem to be water proof. The ones with the remote control are not (found this out the hard way). Also the cost for a chipped key may depend on how many you have (due to programming requirements). If you have 2 already, the 3rd one is a lot less expensive, you only have to buy the key and program it yourself. However, if you are down to 1 key, then you must also pay the dealer to reprogram it, making it even more expensive.