Just got back from a cruise leaving 1/14/12 to Belize. I know you didn't ask, but if you are also entertaining the idea of the private beach then re-think it unless you want to walk on a man made beach full of sharp sea shells. Don't get me wrong, very pretty but uncomfortable on the feet. No sand on the beach.
We did cave tubing and zip line with this company ( Action Boys Belize.com ). It was a great experience. They did't join us with any group We had a private van and a private tour guide assigned to us. This was V.I.P. style. I recommend them.