Not sure exactly, unless you are traveling on NCL. If so, all of the spots are purchased by the cruiseline when they are in port, so that their passengers are the only ones at this point that can do it on those days, at this point. There is supposedly another company being established so that others can do this even if NCL is in port. I am also told that others MAY be able to do it later in the day after the NCL passengers have left.
We just did this on our cruise at the end of Jan, and had a blast. The company is Canopy Tours, but if you are cruising NCL, the tickets can only be purchased through the ship and can also be reserved prior to leaving online. I can try to find out the name of someone you can contact outside of the cruiseline.