We sailed in Belize in February of this year -- using TMM out of San Pedro. I have a full-blown report on my personal website: www.homestead.com/islandtime/MainPage1.html (you can also find it in the Articles section of the Ambergris Caye message board at www.ambergriscaye.com).
I honestly feel like we didn''t get enough sailing, and could have spent many more days there (we actually tried to extend our trip, but the boat was already booked). But in our situation, we missed a day of our trip due to a weather delay on the way down, so missing another day to explore inland would have really cut our sailing time. As well, we had visited Belize a few years before, on a land-based trip, and had the opportunity to explore the Mayan ruins.
Ultimately, it''s up to you and how you like to spend your time. Once you are outside of Ambergris Caye and Cay Caulker, you basically leave "civilization" behind: no restaurants, no shops, no provisioning opportunities (plan accordingly for water, food, ice, fuel), no night life. If you need civilization, you may want to plan a shore excursion.
If you have any more questions, you can e-mail me through my website.