Just came off the Valor last week. We did not go to Cozumel this time, but I've been diving there before off a cruise ship and went with Sand Dollar, so can speak to that.
Grand Cayman: Operation is Don Foster's Dive. There was 16 on the dive boat, which is too many for me. We were separated into two groups of 8 for the diving and the briefing. Brian was the DM I liked the best -- he seemed like he truly loved diving and being a DM. It was an OK experience... the reef seemed over-dived and the staff seemed somewhat jaded toward the regular cruise ship tourists. I'd probably book my own dive next time. Eden Rock was a short walk... maybe 4-6 blocks to the right along the shoreline as you are looking toward the shore from the ship. Don't know anything about their op, but the shop looked clean from the outside.
Roatan: We booked our own. We dived with Anthony's Key from the Valor last time and it was terrible. Forty people on a single boat and terrible equipment. I understand that they split the group this time around and the experience is MUCH different if you actually stay at the resort (sign-up for diving on a board, smaller boats and groups). We booked with Bananarama and it was the BEST dives of the trip. Martin was our DM. The resort has a great beach with a shady bar and live music. Definitely worthwhile. We also stayed on Roatan for a week and dived with Coconut Tree and were very happy with them as well. Small groups, nice service -- we would definitely dive with them again. Just don't stay at the cabins behind the dive shop (they look nice from the OUTSIDE...). Definitely consider diving in Roatan.
Belize: Op is Belize Dive Connection (
Hugh Parkey Belize Dive Connection - Accommodations). The dive boat picks you up and drops you off at the ship (it's a tender port, as is Grand Cayman), so you won't be able to do any other activities or shopping if you dive. It's about 30 minutes from the ship to their private island, then another 30 minutes to the dive sites. The timing meant diving as the tide was going out and there was enough current that our second dive became a drift dive. The viz was poor, the reef was nothing special, and the long distance makes this the most expensive of all the dive trips on the itinerary. I would probably skip Belize.
Cozumel: Dived with Sand Dollar last time we were there several years ago. Small group, DM was Miguel N (Abuello/"grandfather"). I was very happy with the dive and the operation -- one of our top 5 dives (my hubby's #1 ever and my #2 (I liked the shark dive in Bora Bora just a little better)). Cozumel was a drift dive and we had amazing viz. I would book with them again separately or off a cruise ship. I can understand wanting to try someone else to experience something different, but if it ain't broke...
The ships really don't understand divers, but seem to be offering more diving as it's popular. After doing this several times now, I am tending towards booking my own boats. Better if you have a group of 6 or more. Best is to stay on a island if you find something good off the ship and understand that you will likely not be diving the best sites from the ship -- it will be better diving when you stay. We did have some great diving on the Southern Caribbean cruise off of the Victory in January. Especially notable was Dominica... excellent diving.
Have a great trip and I hope this was helpful.