My wife and I regularly sail in Belize. Provisioning in Placentia or San Pedro is possible, but difficult, and will usually require visiting several different stores. Although we regularly provision ourselves in the BVI, we never do that in Belize. I strongly suggest you let Moorings provision for you. With very little effort you would be able to buy fish from the fishermen every single night, and if lobster season is open, (and it should be) live lobster most nights. Liquor in Belize is heavily taxed, and therefore pricey, except for the local rum. It is not possible to get onto the boat without spending the night before in Belize. Therefore I suggest you eat dinner at a decent restaurant, and everybody order a different rum. Choose what you want on board on the basis of your "taste test". We enjoy Barrel One. There are several pleasant restaurants in Placentia. Last month we had a lobster dinner in one, and then two nights later had the same dinner on one of the islands. The price was exactly double. The restaurant in Cucumber Marina is very pleasant. Good place to refill with water and fuel. The showers have hot water, but are in a most run down, mouldy, musty building imaginable, and on the far side of the marina. Head facilities are on this side of the marina, and are clean and pleasant. Service in the marina is excellent and very pleasant. Don't think about going into Belize City.
A word about Tropic Aire from Belize City down to Placentia. They do a good job. They are efficient. As soon as you clear into the country, immediately check in, and go to the departure lounge. When all the seats on a plane are full, they may leave. I have seen planes leave as much as an hour early. By the same token, I have seen them when they over sold a plane, and too many people show up. No problem, they just put another plane in service, and get the passengers there. Have Moorings make your reservations on Tropic Aire. They also take care of passengers when the inbound flights are horribly late. We have NEVER had lost or delayed baggage going down to Belize from Houston or Dallas.
Make sure you know how your depth meter is calibrated before you leave the dock. Have the crew set it for depth under the keel. Then dive and measure the distance yourself before leaving. You will spend a lot of time in very thin water.
I think the most beautiful water in Belize is that between middle Long Cay and the reef. Freya says there is only one channel there, but there are at least two.
Hope this helps. Don