Would you leave your expensive camera sitting on the seat in plain view in a dark, unlit, fairly low-traffic area, where no one would hear or see anything if someone decided to steal it? I sure as heck wouldn't in LA! I hear lots of stories about someone's stuck with a smashed window & a hefty bill because they left their ipod on the dash. It doesn't happen to everyone, but it is common enough so I hide my GPS when I leave.
When we were there last week I was initially concerned, but after experiencing the island firsthand I realize that it's more about common sense. Many of the dive sites are isolated, you're underwater for quite awhile, and if someone decided to help themselves to something from your truck it would be very easy. We didn't see the need to go to such extremes as bringing our sandals underwater, but we made sure that there were absolutely no temptations visible. I had a cellphone with me, but left it under the seat, out of sight. We left the windows open mainly cuz there was nothing to steal, but partially because if someone did break in, we didn't want to have to deal with paying for damages to the rental company.
You don't "have" to do anything... you don't have to leave your doors unlocked, and you don't have to hide your expensive items. But, why risk ruining your vacation if something does get stolen?
btw, we also fly with our gear in a regular, non-descript suitcase instead of an obvious scuba bag. It's the same concept - low chance of the baggage claim people stealing our expensive equipment, but why take the risk?
