Nassau Tour. Prebook or book at dock?

[B&P!!!]

My April cruise is heading to Nassau and I would like to do a custom island tour. I'm thinking about 3 hours to visit a few stores, Atlantis, local sites. Is it best to book in advance or at the dock? I've done it both ways on other cruises. What is the going rate? There would be 4 of us. 

[uCruiser]

We will be doing something lake that as well. Please let me know what you come up with.