MSilvia:There's a reason they don't have a dive operator in Belize.
When I did the Norwegian "Texarribean" cruise in January, there was a diving excursion in Belize. The boat left seven of us stranded in the Carribean for about an hour and a half (not including the 35 minute dive) before we were picked up again.
Needless to say, they refunded our money and dropped the dive op from the excursion list.
We do have an operator in Belize now. We used to use someone called I think it was Aqua Divers or something similar and yes, several people were left by the boat and so NCL stopped using them. Since then however, we have picked up an operator called Hugh Parkey's Dive Center. Maybe www.belizediving.com. Anyway, all we have ever gotten as far as feedback on these guys is great, excellent, and etc. These guys are well organized, very friendly and very knowledgable. So now NCL has dives in all three of those ports. I take it you are going on the Norwegian Sea? I was on the Wind and we did a similar run, replace Cancun with Grand Cayman. If I were to dive just one of those spots, it would be either Roatan, or Cozumel. I dove in Cozumel and one one dive saw a nurse and tiger shark. So that's Cozumel and in Roatan the reefs are beautiful and the island itself is a beautiful island. NCL uses Anthony's Key Resort on Roatan, The Snorkel Center on Cozumel, and Hugh Parkey's on Belize. I'm not sure about Cancun because I haven't been put in that port yet but I can find out if you'd like.