The ship leaves port at 5:00? What time does the excursion end. Just keep in mind, if you go on your own and its running late, the ship won't wait.
The tour leaves between 12 and 1:30 and goes for 3 hours. And it leaves from shelter island which looks to be a 15 min drive from the port. I would love to use one within walking distance, but this one is $200 cheaper than all over the other charters I asked for quotes.
So I guess if the charter leaves any later than 12:30 I just won't do it.