I drive a heap of junk that nobody in their right mind would want to break into. It only has dumb keys, one of which I carry either inside my drysuit, or clipped to my harness. I try not to bring much extra stuff, but I leave it all in the car, tanks too. I'd rather have the key clipped to something than just in a pocket. I'm seriously considering one of those waterproof boxes, though, so I don't have to leave my wallet in the car.
I recently did a dive where we carried our bins of gear down to the beach, suited up there, and left the bins unattended on the public beach. It was my buddy's idea. Nobody took anything, but I didn't have anything very valuable in there, either. Personally, I'm not comfortable leaving anything unsecured anywhere anytime.
When the water's cold, the weather's warm, the distance from the car to the entry point is long, the entry point is at a public beach, and you're wearing a drysuit, there are no easy choices.