Have you encountered this type of key in Bonaire? While I've seen this type of key elsewhere, I've never seen them on Bonaire dive truck rentals. In 11 trips to the island we've never had a rental company ask us to keep their keys dry. However we have had several request that we rinse them after diving. I continue to clip my keys inside my BC pocket.
When my wife is diving with me she carries the truck keys and some cash in a small Otter box like you described, also clipped inside her BC pocket. These little boxes are great. We use the OtterBox Drybox 1000. It's rated to 100' and only costs about $12.
I have last summer, it was a regular key but had an additional fob on the key ring. My solution was to remove the fob and lodge it in an inconspicuas place under the dash near the ignition key hole and take the key with me. Worked fine all week and I put them back together at the end of the week.