Wow... seems like a lot of negativity for Belize. I'd like to offer some counter viewpoints since no one seemed to be in favor of Belize.
While I agree that the city beyond the fenced in area doesn't look spectacular, its not really unsafe. Would I wonder around alone with money hanging out of my pockets? No, but I would do that in my city either. I have left the fence area a couple of times and to be able to do anything beyond shop in the expensive cruise line provided stores in the port area you have to leave the fenced area. Yes, it is a poor country but that doesn't make them all thieves and rapists. I have met some wonderful people in Belize. I have NEVER been offered drugs, so unless you are asking for them, I wouldn't expect that.
The water... yes, the water looks brown and dirty... but realize the port area is at the mouth of the Belize river. So what you are seeing is the tidal effects of the river coming into the ocean. Yes it is dirty looking. No, there are no beaches near where the tenders dock. BUT Belize does offer some of the best diving and snorkeling in the Caribbean. The reason the ships have to tender you ashore there is because they have the worlds second largest barrier reef system. (Might be third, but I am sure it depends on who you ask and how you measure it). Next time you are there, take one of the snorkeling tours. They will pick you up from the ship and take you to the reef. So you don't have to tender in at all. Its amazing. If you are a diver I probably don't have to tell you to take a dive tour. They offer several beach island tours where they took one of the barriers reef islands and converted it to a beach "resort" They are very nice.
If you are not a beach person take a tour (ship provided or book one on your own at the tender dock or on line) They have an amazing cave tubing experience and the have animal parks (monkeys and jungle life) and they have Mayan ruins to see.
Belize is far from the crap hole that the others have painted it to be. I won't get off the ship in Jamaica due to the bad experiences I have had there, but we always go into Belize. Unless your only reason would be to just go to the shopping area in which case, yes you may as well stay on the ship. The merchants in the port area know they have a captured audience because no one ventures outside (not that there are stores to shop at beyond the port area) so they rarely barter at all. If you do go beyond the fence (which is really there to keep the local people from getting the tourist business as it was built by the cruise lines who built the shopping mall) you can find local artists and others you can buy true locally crafted items from.
So don't be so quick to pass the place up.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013