Welcome to ScubaBoard,
I took a Carnival Cruise to Grand Cayman, Belize, Roatan and Cozumel in Feb 2008. Here is what I can tell you:
1. Leave the dive knife at home. The cruise line will not let you have it on board and you don't need it anyway. Take some Shears for your emergency fishing line cutter (never needed that either but you might). If you must take your knife the cruise line will take it from you and you can check it out on each stop, big hassle.
2. Take a 3mm Wetsuit or at least a rash guard. I get cold easy and the F water is great, but many of the shorts only crowd was cold by the second dive, I was not. The sun will also fry you without something on even in the water. Good to have 3mm on your if you accidentally brush against something like coral. Ran into a bunch of very small jelly fish on one dive. I actually only saw one, but my face saw several. If not for the suit, I would have had more exposed skin.
3. At all my stops dive gloves were not allowed to keep you from touching things like the fragile coral (a light touch will kill it).
4. Sun screen kills the coral. Many places will not let you use it, or require it to be a special type that will not form an oily blanket on the coral.
5. Bring a hat, large brim and of course sunglasses.
6. Leave the large SCUBA suitcase on the boat and take a small bag with your stuff in it. The dive boats are rather small. My regulator, towel, backplate, wing, dive computer, booties all fit in a nylon bag (I bough one from the cruise ship) of the kind often given away by say PAID or DAN when you renew. I then routed the shoulder strap through my fin strap and that worked just fine.
7. I took all my own gear (back plate, wing (BC), regulator, computer, fins, wet suit). That meant that after each dive I had to clean my gear and dry it (balcony or shower). All the other divers rented their gear and at the end just tossed them to the dive operator and went off to do other things. That said, having seen the condition of the rental gear (torn, leaking, etc), and an eye witness account of a Caribbean dive shop involving a regulator and clogged toilet...., I am very glad I took my own gear. I would recommend it. They will provide the weights of which, with a 3mm suit) you will need 10 pounds or so.
8. If you take your own gear, take a written list of serial numbers, manufacturer and model as well as a picture or two. Comes in handy just in case you leave something behind. I got rushed at our last stop and left my regulator and computer hanging over the rinse tank. They of course could not find it until I told them I had the serial numbers and was calling the police in one hour. Miracle, they found it in less than 5 minutes and mailed it to me!
9. The cruise line dives will cost you $40 or more than what the local dive operators charge. Advantage, the dive boat does not depart until you get there and the cruise ship will not leave until you return, or if it does the cruise line is responsible for getting you to the ship since you booked through them. At some ports we were tendered (anchored off shore) and the dive boat picked us up at the ship. Disadvantage: You will only get 2 dives in each port and it cost a bit more. Given the limited time in each port, I doubt you would get more than 2 dives in any port. We usually had 2-4 hours (3 was the norm) to sightsee after the dives.
10. Book a dive at the first port it is available. You may quickly decide (I bet) that you want to dive each port. The water is warm, you can see 80-100 feet in clear water and there are lots of beautiful colorful fish to see. Much more interesting than a quarry or lake dive. I dove all 4 stops and only wished I could have done more than 2 dives at each stop.
PS: if the ship design is such that the outside cabins are on the edge of the ship (no public walkway between your cabin and the rail) book the corner cabin on the stern. Cabins are the size of two bunk beds side by side and two night stands wide and only a few feet longer. Corner cabin is 1/2-3/4 bigger. Balcony cabins get 10 feet of balcony, corner cabin I had was 13 YARDS long. Worth the small bump in price. We booked the 6:00 feeding. That allowed time to see the shore tour and once the ship left port, shower and dress for dinner, finished dinner just in time to catch the 8:00 show and then in bed by 11:00 (OK, yes, I am in my S). The later feedings had to either skip the shows or do the 10:00 p.m. show and were up late. The SCUBA excursions leave early and you will be off the boat first thing once they dock, so it is breakfast at 6:00 a.m. or so.
To quote an Island local, "Do you see a diamond mine here? So why you come to the island to buy diamonds and other things we do not make here? Buy something we make." Makes sense to me. The ports are full of stores selling stuff made in other places that are not really bargains. Don't forget the US DUTY TAX. You will be taxed on the value of the diamonds etc anyway so once you add that in, you are better at your local jewelry store (where you can see them if something is not up to standards later). The soda cards are a bargain only if you drink 4 or more a day. You are SCUBA diver and should be drinking water anyway, and tap water is just fine thank you. On a cruise your on board bill (they charge you a standard rate tip, port charges, excursions and of course your bar tab and gambling tab) will often be more than the cruise cost. We did the whole 8 day cruise, excursions (wife sightseeing me diving in each port) with corner balcony stern cabin for under $5k.
Good luck and enjoy your cruise. I hope the above helps.
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