I don't really understand the issue, is it not just as risky with a normal key?
I thought I had understood it but I might be wrong.
Normal key has no electronics so can be taken into the water where it is on your person at all times. Presumably it goes in a water proof pouch, but this isn't 100% guarantee, keeps rust off though.
Because it is not 100% guarantee its too risky to take a keyless key into the water... But you also can't hide it near the vehicle or in the vehicle or anyone can gain access and pinch your stuff. If you can block the keyless signal with a metal lined pouch then you could conceal the key near the car safely (unless someone finds it!).
... Did I get that right?