I carry my key on me in my aqua pack with the gps - never had an issue with water getting in. Makes it nice and eaasy to get in and out of the car just press the lock button on your arm then relock it once done.
Pardon my stupidity but do you just hook it up to the tow hitch? In plan site? I normally take a spare key in my wet suit or boardies pocket but atm only have the genuine key with remote. I just had extra keys cut for my old Prado only to trade it in tow weeks later.