If you still have an old key without electronics, you can simply hang it around your neck. However, there is always the risk of losing the key on the water - and then standing wet in front of the locked car. It is better to leave the key on land, but everyone else knows most of the usual hiding places.
With the Key Sage from Abus, you can go on the water worry-free, your keys will stay dry and safe in the box. All you need to do is memorise a numerical code and hang the safe on the towing eye of your car or somewhere else suitable!