Right after the ships tours depart. They have priority. Understand that while many private tours are a little cheaper you are really on your own with them and have no recourse if they do not provide what they promise or if they rip you off. It isn't done often but it is done and they know there is nothing you can do about it as you are leaving on a ship that day. With a ship sponsored tour they can take care of problems and have influance on the operators. In addition if anything happens at least a ships tour will get you to the ship or hold it, not so with independant. ALso ships tours have insurance, again, not so with independants. Just know what you are doing and getting into is all I ask.