Not sure where you are staying so not sure what you will pass but Le Grande Marche on Bush Rd is very popular and has a decent deli. In the Simpson Bay area Market Garden is not bad either.

Not sure on the other question but I have seen basic snorkel equipment at various places around the island, just never had a need for it.

Whatever you do, do NOT leave anything of value in your rental car (assuming you have one) as that is a recipe for disaster.

Good luck.