Chankanaab is on Cozumel and Captain Marvin is on Grand Cayman. Both are reputable and I have never heard of anyone not receiving a refund in the event the ship is unable to make the port stop.
Grand Cayman is the port that is most likely to be missed because of bad weather.
Chankanaab is about a ten minute cab $6/pp cab ride from the dock. Most of Captain Marvin's tours pick you up near the dock or just across the street.