Just did this trip last week. At Belize you will use Hugh Parkey if doing a ship's excursion, which I highly recommend doing if for no other reason it is a tender port and the dive op picks you up and returns you directly to the ship. We had a bit of a problem because the RCCL shore excursion staff put us on the first tender to shore (20 minutes in the wrong direction), only to have to return to the ship where the Hugh Parkey dive boat was waiting for us. The dive boat takes you to Parkey's private island where they load up the tanks and dive crew and off you go to Turneffe Atoll. The two dives at Turneffe Atoll were some of the best dives I have taken using ship's excursions or outside vendors. The dive crew was very professional, the boat was very good, and we dove down to @500psi, so we did not get shorted on dive time. They had some snacks for the surface interval, and even served up a lion fish killed by one of the DMs, which was chopped up and put in a jar of salsa. Quite good for raw fish marinated in salsa. The reefs were gorgeous, and I cannot wait to go back to Turneffe Atoll. After the dives, and enroute back to the ship, we stopped at the dive shop on their private island so the crew could off load the tanks, and we had time for an adult beverage, and bathroom break. Delivered us back to the ship with plenty of time to spare, even considering the 45 minutes wasted while RCCL sent us to shore instead of waiting for the dive boat. If they try that with you, tell them to go get a supervisor because you know the dive boat is supposed to pick you up at the ship. Have fun and enjoy the dives. Belize is one place that you really do want to use the ship's excursion because there really was no downside to doing so, and if you are not booked on a ship's excursion, your boarding priority for boarding the tender will be after all their shore excursions have departed.
---------- Post added August 22nd, 2014 at 11:06 PM ----------
I forgot to mention that there were 22 divers on the boat, but the crew handled the load extremely well and were exceptionally well organized. They broke the divers up into three groups of 6-8 based on experience level, and then the first group geared up while the other two groups were getting their dive briefings on the front and top deck of the boat. After the first group hit the water, the second group geared up, and the cycle was repeated for the third group. As a result, they did not have divers tripping over each other donning gear or getting to the back of the boat for water entry. They do not allow walking on the boat with fins, so everyone put the fins on just prior to the giant stride off the back of the boat. It was organized, but a very casual and relaxed way to get the groups in the water. The groups came back pretty much in sequence so the first group was out of the water, tanks changed out, and snacking started before the second group was ready to exit the water. Same process for the third group. Second dives were set up the same way. Overall, I would rate their dive op as excellent, and would not hesitate to use Hugh Parkey and the RCCL ship excursion next time during a cruise stop over.