I have never found any really good, high-quality Belize maps on the Web. There are some, but they're all lacking in some way or another. The best general map to the country is the Belize Traveller's Map from ITMB, and it does show the reef and many, not all, of Belize's 200 or so islands. It's US$8.95, plus shipping. We sell it, but it's available from other sources, too.
There are a quite a few places where you can snorkel from the shore, even on Ambergris Caye and Placencia. For example, in Placencia Kitty's has some fair snorkeling areas and on Ambergris Caribbean Villas has a little artificial reef area by its dock. As long as you're not in a busy area such as the water off Front Street you can pop in and snorkel. It's not like being on the reef but there is a surprising amount of sea life.
Islands on the reef and/or with coral heads nearby are mostly small -- places like South Water (which isn't all that primitive), Tobacco, Goff, Rendevouz, Northern, Little Water, English, Half Moon, Ranguana, Laughing Bird, French Louie.
Some of these have lodges or houses, some don't. But all are pretty isolated and offer little except water sports. One of the guidebooks such as Moon Handbook or Rough Guide can be of help.
I know what you mean about getting back into a boat. I have a touch of arthritis and am getting fatter by the year, so it gets harder to embark the bark. But it depends a lot on the boat -- some are set up for ease of getting in and out, and others aren't.
Belize First Magazine http://www.turq.com/belizefirst/