Have a great cruise! My family and I were in Belize last winter and didn't do an excursion. I would NOT recommend this. :-)
There are tiny shops a-plenty, but they'll bore you very quickly. Do eat at the Wet Lizard if you get the chance--try the conch fritters!
We ended up back at the ship pretty early--just after lunch...but it was fine because there was plenty of room at the pool and bars.