Get one ^ of these.
http://www.leisurepro.com/catalog.aspx?op=gcssearch&term=witz+case
Not sure I'd trust the phone case but I've used one of the thinner ones (same one) for a decade with no leaks - including a week of shore diving on Bonaire. It has a small o-ring seal that I lube once a year or so. I actually wear it on the lanyard most of every day while diving/driving around tucked under my shirt.Cash, credit card, sometimes c-card if I think I'm diving with multiple shops.
Otherwise show your BMP orientation tag (it clips to your BC) on the off-chance you encounter a BMP ranger - we never did all week except once on a boat dive briefly. Our DM told us she would be allowed to go home to get her driver's license also but not sure if that applies to guests.
They also sell two versions of a glasses case. I don't know how the wider phone/glasses cases will handle the pressure since none of these are dive rated.
Some of the car rentals on Bonaire are rented with smart keys now so the carbinier/quick links in a pocket are not such a good idea.
Although quick links do work well, I use one to mount my video housing coiled retainer to my BC d-ring. Since they lock down, not much chance of them coming loose.
Here's what we're talking about but I also buy the plated ones from Home Depot.
Quick Links : Tech Diving Limited, The Leading Source for Diving Equipment
Almost all the trucks are 4 doors so if there's 2 of you, the backseat is a good place to loosely store your gear between dives. Tanks go in the back and won't be touched, no shop fills anothers so they're not good for resale.
I'm not suggesting this but I left my video camera between the seats in town covered by a towel accidentally once while we ran an errand - windows down. Still there when I got back.
I think most of the thefts occur at the remote sites where they can watch from the bushes/rocks and know you'll likely be gone 45mins. to an hour.