yes, a lot of this does depend on who you are diving with. are you using your own equipment or renting? if renting, you will need your own mask, fins & snorkel. if you have your own equipment, some dive ops valet your equipment overnight (rinse, store and set up on the boat the next day), others want you to handle your own equipment, which means you have to carry it to the boat daily ... we have mesh duffel bags we bought at a sports/camping store (Acadamy or a place like that), we put our equipment in to take it to the boat, the mesh bags fold up into almost nothing when empty and store easily on the boat.
it's good to have a few bucks with you, the dive ops I use usually don't stop at places with facilities, people either pee in the woods or in the water, but we do go to the "Caletta" (boat harbor) sometimes and they want a dollar to use the bathroom there .. I know, right?!
some dive ops will stop at one of our 'day beaches' for their SI's, so having a few bucks would help there also, some even eat lunch between dives at these places.
depending on where you are staying and what dive site you are going to, your boat ride to the dive site could be anywhere from 15 min to nearly an hour. The Marine Park and most of are reefs are stretched out along the length of the island on the channel side, with the "wall" being about 1/4 mile off shore at the most. The island is never out of site.
most dive ops pick you up at your hotel pier, so leaving your stuff at the dive shop may not be an option. I wouldn't do that anyway unless they have lockers you can lock.
I'd check with who you are going to dive with and see what their SI policies are, if they pick up at your hotel, and any other concerns you might have. Most dive ops and hotels are very willing to help you out as much as possible.
a couple hints in case you haven't booked yet. MOST dive ops DO NOT pick up in the northern hotel zone, so staying in town, (you will have to walk to a downtown pier if you do this), or south of town is your best bet of not having to take a cab to your dive pick up in the morning. Again, most dive ops pick up at most hotel/resort piers from town, south.
Aldora Divers is the main exception to the 'northern hotel zone rule', their lodging, Villa Aldora is located north of town and is a very nice property and the Aldora dive operation is an exceptional, well respected dive op.
feel free to PM me if you have any questions.