My wife, son, and I vacationed in Placencia, Belize last December. It was a fantastic vacation. We did about 30 dives with Splash dive center, and they have my
highest recommendation. Very professional, very safe, great group of DMs, they were just the total package and we loved them. I found the reefs in Belize to be less spectacular than Cozumel in formation, but in better health due to a much lower density of divers I believe. I didn't see near as much damage as I normally notice on a Cozumel reef. But you won't see many of the spectacular coral formations Cozumel is known for. The schools of marine life were about the same as Coz to my untrained eye. And we were almost always alone in the water. The only time we ever saw another group of divers underwater was when we went to Lighthouse Reef and the Blue Hole, which was one day out of about 10 days of diving.
We did do the Blue Hole dive and it was pretty cool, mostly because we had a bunch of 6 foot reef or lemon sharks swimming very close to us down around 100 feet. Otherwise there is not much to see and even the formations and overhangs will be a bit underwhelming if you have ever done any cenote diving. As I have heard repeated many times in regards to the Blue Hole, "I am glad I did it but probably won't do it again". The 4 hour boat ride each way from Placencia was enough to discourage that.
And that takes me to the only downside of diving from Placencia. You will be on the boat at least 3 hours a day, and sometimes as much as 8 if you are going to the Blue Hole and back. There just aren't any really good reefs less than 90 minutes from town. A couple of those days were a bit rough, too, so if you are the seasick type you better cross your fingers it's calm while you're there. One day we had 6-8 foot seas out on the open ocean and that was no fun at all, even in a 46 foot boat with padded benches. Most days though were very calm, and we even learned how easy it was to take naps on the boat as we got used to things.
It was a great trip. Placencia is exactly as it is named - very pleasant. Unlike what I have heard about Ambergris / San Pedro (see eab's post above mine for an example) I felt Placencia was a fantastic little town and felt perfectly safe there. There were plenty of nice restaurants and even a fantastic Guatemalan coffee house, Above Grounds, owned and operated by a charming man from... New Zealand I think... if memory serves (and it often does not as I grow older). Oh and the gelato shop in the center of town is NOT TO BE MISSED UNDER ANY CIRCUMSTANCES. I also had the best breakfast sandwich of my life at the Swiss bakery. That was the surprising thing about Placencia to me... everyone there was from somewhere else! Swiss, New Zealanders, Italians, Germans, a couple from New England ran our hotel, and we even met an older couple from Texas that run a pancake house. It was very international for such a small town!
Many, many good memories of that place. My recommendation - do yourself a favor and go there once in your life. I have been a lot of places in the world and never seen any place quite like it.