My family and I just returned from a 10 day trip to Ambergris via Belize City. We stayed at Banyan Bay, which is well run, and I am comfortable recommending it.
Ambergris Caye: if you are going to SCUBA, you are in the right place. The reef is awesome! If you are new to the sport, try to find the old man named Sylvan, just ask for him downtown, and you will find him. He parks his boat by Bottom Time Dive Shop. He is an A+ quality instructor, and is great with kids.
The beaches: I would rate the beaches as a 2 out of 10. They are very narrow, man-made, and full of sea grass. Swimming in the ocean is limited at best. I jogged up and down the beach from San Pedro all the way to where the resorts end about 3 miles south, so I think I saw most of the beaches. Banyan Bay and Ramones had the best ones, but most of the resorts had decently maintained beaches. Toplessness is NOT ok on the island, which my wife found out on the first morning when a surly security guard informed her that it was "illegal". It was funny because some of the fat men on the beach were sunbathing their large breasts with no complaint. A double standard for sure, but we respect the cultural moras of the places we visit, and she complied. Don't go to Belize for a beach trip, there are many, many better beaches in the world, and in C.A. for that matter.
Food prices were ridiculous by central american standards. A hamburger at Banyan Bay was $8 US, a price you would expect at a luxury resort in Hawaii, but in C.A.? The local restaraunts were about the same price. There is little strip of "food stalls" in downtown San Pedro where you can get a decent taco or burritto for about a dollar, so there is a cheaper option if you are OK with eating on the street. I am. We bought a lot of our own food at the grocery store in San Pedro, where it was also high priced. In fact, everything is expensive in Ambergris, which for me takes away one of the great qualities of travel in C.A.
The nightlife is OK, my wife and I like to dance and there was a lot of dancing to reggae bands on the beach. We had a few uncomfortable encounters with native kids at the bars. My wife is admittedly attractive, and some of the rasta kids were a little agressive when dancing, and had to be told very directly that it was OK to dance, but no touching or dirty dancing on her behind! She had to yell at one boy to get him away, which was a real drag. Fortunately, I never had to make a John Wayne style intervention.
There is basically no police presence on the island, which can be unnerving at 1 in the morning. That said, I typically would not be in bars at 1 in the morning in C.A., but Ambergris is safe enough to allow it with caution in mind. Earlier in the evening, we never had anything but polite exchanges with both tourists and natives. It was only late at night after the alcohol had been flowing that we had some small problems.
Drugs: like anywhere in the world, you can find drugs on the island. On a few occasions, a kid would ask us if we wanted to buy some ganja, but there was no harrassing.
Alcohol: Rum is very cheap, and it was all good. One bad thing about Belize is that there is a beer monopoly. The beer is called "Beliken" and the only worse beer I have had is Guatemala's "Gallo". That is a drag because when I am on an island trip I like to drink Corona with lime, which I know makes me the ultimate gringo, but I like what I like. So BYOB if you like beer, you cannot get any other beers in Belize, even at the liquor store.
On a final note, I noticed some contradictory information about Belize City. Belize City is dirty, poor and dangerous, and that is not coming from cruise ship tourist that gets frightened around poor people. It really is a mess.
In Summary: Ambergris Caye is awesome for SCUBA. If you want great beaches, go to a carribean island with natural beaches or Playa del Carmen, Mex. If you want culture, go to Antigua Guatemala or Mexico City.