Strong proponent of having a key copy (without electronic or sensor bar in them) to open and lock your car doors on you while you dive, while keeping your ignition keys hidden in the car somewhere.
On a side note, think twice if the local dive shop near the shore diving site offers to hold your keys while you dive. Had one instance where the shopkeep forgot he was holding onto my keys while on a shore dive - closed the shop early and was not picking up his cell phone (that his neighbors in the strip mall called for me several times) - had to call a friend to pick me up and drop me off the next day to pickup my keys & car....
On a side note, think twice if the local dive shop near the shore diving site offers to hold your keys while you dive. Had one instance where the shopkeep forgot he was holding onto my keys while on a shore dive - closed the shop early and was not picking up his cell phone (that his neighbors in the strip mall called for me several times) - had to call a friend to pick me up and drop me off the next day to pickup my keys & car....