Simple rule of thumb:
Don't leave anything in the truck that you wouldn't willingly throw away.
I think the one thing that first-time visitors to Bonaire don't appreciate - I know I didn't the first time - is just how small the island is, and how easy it is to get around. Look at the circle I've drawn around the Dive Hut on the map below.
There are
58 dive sites within a 5-mile radius of where you're staying!
In fact, with the exception of a handful of sites up in The Park, you can easily drive
from any dive site on Bonaire... back to The Dive Hut, swap your tanks, get fresh batteries, take a shower, change your clothes, make a few sandwiches... and then drive
to any OTHER dive site on Bonaire... and still have 20 minutes left on your 1-hr surface interval! No need to bring tools, extra batteries, changes of clothing, a giant cooler full of food, etc, etc.
My buddy and I leave a few bottles of water and a couple of sandwiches in the truck. And we don't even leave them in a cooler. We leave them in a bucket of ice water... because petty thieves won't take the time to open a cooler to see what's in it. They will simply grab-n-go, hoping that you also stashed your wallet or something else of value in the cooler. This is why you hear of people having old tee-shirts and ratty shorts stolen. It's not because the thieves want your old clothes... it's because they're hoping you left $10 in the pocket.
I put a Visa card, my ID, the truck key, and a few bucks in the zippered pocket of the board shorts I wear into the water. I don't care if the first three get wet... and any merchant on Bonaire that won't accept wet currency (or wet patrons, for that matter) will be out of business within a week.