Shipwreck & reef w/ Capt. marvins (NOT through
DCL) went to Eden Rock (IMO deep, not too much to see), Callie shipwreck (shallower, neat to see shipwreck but not too much sealife), Cheeseburger Reef (honestly don't remember too much about it but not too many fish), and one other shipwreck in front of 7 mile beach Westin hotel I think? That one had the most sea life and most recent ship so more to see. Further out, deeper water.
All were fairly calm, but it's not water you can stand in by all means.
Eden Rock, Callie are right in the heart of town and you can literally walk to location on land where they have signs Callie, or Eden Rock..., get in the water, swim out to them. Cheeseburger reef was right out from Burger King I believe, but is further into the left of town (if coming off the pier). Probably able to walk to it, and swim out to it. 7 mile beach wreck was about a 15 minute boat ride down and was further out probably not swimmable to in my opinion.
Our trip was about 2 hours I think, with about 15 minutes or so at each of the 4 stops. There were only supposed to be 3 of us on the entire boat, but at the last minute 2 people staying at a hotel signed up. So, it was a total of 5 snorkellers and Chris from Capt. Marvins (who stayed on the boat and threw bread out to feed/attract fish) This trip is not too exhausting, and easy snorkelling (IMO). We did this one back to back w/ the morning 10-1 one. We are experienced snorkellers and can tell you we made it to dinner and the show (even Palos that night I believe) but we did sleep very well that night!
Stingray city and Barrier reef on other side of island were much, much more exciting w/ sea life, but that might not be possible w/ your time frame?
Hope this helps!