Waterproof key

I need a key I can take in the sea with me. Can I get a basic key cut without any buttons etc and will it work? I’ve tried numerous waterproof boxes etc and never found them watertight.
The easiest way to cover this aspect is to get a basic blade/key cut that would lock/unlock the car given that you have the mechanical door locks, at least in the driver side. To start the car you will have to keep your original key along with the house keys in the car somewhere.

To start the car, you will need a key with a transponder, but to lock/unlock manually, you can use a normal simple key. Blades can surely be found on Ebay then visit the local key cutter it have it done.