The inevitable end of all your possessions while living aboard is "falling into the water". At some point, everything that you cherish: tools, keys, phone, ect. will fall into the water and be lost forever. The trick is to postpone this inevitable end!
Keeping your keys on a lanyard around your neck is a very secure way to make sure you remain in possession them. It might not look stylish, but this is the safest way too keep ahold of them.
It doesn't take long for a new liveaboard in the marina to start carrying they keys on a lanyard around their neck. Maddie started almost immediately, despite her resistance based on fashion reasons.
I use the BoatUS lanyard that came with my insurance policy and Maddie uses a lanyard from college. These simple lanyards have kept our keys attached to us for years, and hopefully the day they fall into the water never comes!