We just did Capt. Marvin's 1/2 day tour last Friday. We had to be there by 9:15, we were heading out to the first spot by 10:15 and back by 1 P.M.. It was great.
There were snacks on the boat, as well as juice and water. We decided to grab some sandwiches from the bakery right down the street for the trip and that was a great idea.
We had time enough to hang out at the beach for awhile as well, afterwards. I would go to one of the hotels closer to the port as opposed to the public access beach ( we got stuck in major traffic - although that is supposed to eb unusual ) but did make it back to the ship in time for the 4:30 tender.
We booked through the website with no problems.
Have fun -