If I remember correctly, San Pedro is the main town/city on the most popular (possibly largest) of the cayes, called Ambergris Caye (cayes are islands, like the Florida Keys - pronounced the same way "key")
San Pedro is close to the south end of the caye/island with a ton of resorts sprinkled up along the coast. It is primarily visited by scuba divers, since (as someone else mentioned) the barrier reef is only about 200 yards off-shore. The diving is great, so if you guys are scuba certified, you can make that part of your honeymoon. There is even better diving on the smaller atolls, a few hours further out - like Turnefe and Lighthouse Reef.
San Pedro is pretty safe, even in the evenings - but I've heard that inland can be somewhat iffy, and that the capital (Belize city?) can be a bit dangerous.
On the other hand, day trips into the various Mayan ruins are really, really cool - and there is lots of neat wildlife as you take boat trips up the rivers.