Hi,
I just looked at their website and things have apparantly changed a bit since we went.
We went on the Stingray City "tour" first thing in the morning, and stopped at ONE other snorkel site. I don't remember exactly how long it was, but a few hours...
We were done in time to then go on a second snorkel trip with them, one called a "Shipwreck Snorkel". That was a shorter trip, and made 2 snorkel stops.
Of the two tours, the Stingray City one is/was by far the best. The Shipwreck didn't really have much marine life around. If I could do it all over again, I would do whatever is in my power to do Stingray City ALL DAY!!!
They are VERY accomodating and will understand if you explain you're on a ship, etc. I'm sorry I can't help more, but it looks like the times and variety of trips has changed some.
We were able to easily walk to their location in town, where they pick you up and take you to their boat. (We were walking with a 3 and 5 year old so I promise, an EASY walk!!)
Hope this helps a tad bit. TKB Lisa was on our stingray city trip, she might possibly remember the times....