+1 on the Brita, lots of canned meats, label maker, Sharpies, and the email to guests telling them what to bring (and NOT bring!), and what to expect. We actually tell our guests to purchase travel insurance, on the off chance that something goes wrong on our end that means they can't come.
RE: dinghy theft - we spent 8 months all over the Bahamas and never locked our dink, and never heard of anyone having a dink stolen. We left it tied to the swim platform every night (unless we hoisted it onto the deck for an early a.m. departure), and for one three month period (when our Steelhead crane was inop) it was tied up every single night. I'm sure dinghy theft happens, and it has to happen only once to you to ruin your day (and maybe your trip), but I wanted to comment on your comment that dinghy theft is fairly common in the Bahamas. That's certainly not been our experience.