I'm posting the relevant excerpt from the US Government site referred to above:-
Water Safety: Boats serving the public, especially water taxis, often do not carry sufficient safety equipment. Many carry an excessive number of passengers and may sail in inclement weather. Rental diving equipment may not always be properly maintained or inspected, and some local dive masters fail to consider the skill levels of individual tourists when organizing dives to some of Belize’s more challenging sites. Deaths and serious injuries have occurred as a result of the negligence of dive tour operators, the lack of strict enforcement of tour regulations, water taxis diverging from routes when tourists are in the water, and tourists’ neglect of their physical limitations. The Embassy strongly recommends that anyone interested in scuba diving or snorkeling while in Belize check the references, licenses, and equipment of tour operators before agreeing to or paying for a tour. The Embassy further recommends that U.S. citizens be forthcoming in reporting pre-existing medical conditions to their dive tour operators, and comply when a dive tour operator prohibits participation in such activities due to a U.S. citizen’s health condition. Safety precautions and emergency response capabilities may not be up to U.S. standards. All tour guides and boat captains are now required to be licensed by the Government of Belize. The only hyperbaric recompression chamber in Belize is located in San Pedro Town, Ambergris Caye.
---------- Post added December 10th, 2012 at 03:43 PM ----------
That advice includes some indisputable fact, some disputable "fact", and general warnings that would apply anywhere. People should remember, in ALL their dealings in Belize, that there is often no "long stop" legal protection. In particular, financial fraud is widely practiced and there is little protection in law once you've handed over your money. "Caveat Emptor" applies more in Belize than in any other country I've visited.
That said, if you bear in mind that water-based activities are inadequately or not-at-all controlled centrally and you must assess each operation individually, there are many safe and honest such operations in Belize. There is nothing particularly unusual in that. In Belize if you burn the inside of your mouth on a cup of hot coffee that will be the end of it - there is no possibility of "sueing" anybody. When diving you must take primary responsibility for your own safety, just as in fact you must in Mexico. If you're taking a tuition course in diving, DO check the credentials of the instructor and of the overall operation. If you have any misgivings then either check directly with PADI (if it is a PADI-sanctioned operation) or with other local operations. Try to discern the normal local cost for an activity/course - if yours is much cheaper then you can be sure corners will be cut. In general, in Belize and all other unfamiliar countries do NOT automatically go for the lowest price - you'll get at best what you pay for. At the same time, use your eyes and your wits - if for example air is leaking from a scuba tank at places it shouldn't, don't just reject that tank but ideally don't use any tanks filled by that operator. If that means you don't dive, so be it. Diving is a matter of personal security, and YOU must take care of yourself - only defer that care to others if you feel totally at ease doing so.
Many, indeed most, people visit Belize and have successful and enjoyable diving vacations - it's not all bad!