I think that's a great idea! Just make sure u have your guests sign up as soon as possible. The cruises fill up quickly. Make sure people wear shorts too bc u have to walk an hip deep into water to get into the boat.
This is a GREAT idea as I did it myself as well in November. My hubby and I got married at 10am, took photos, ate lunch, and hung out on the beach with our friends until 3 when the cruise took off. It was SOOOO much fun, everyone was congratulating us, singing, and drinking of course and it was such a great time! Then we went swimming in the caves and watched the beautiful sunset on our way back in! Plus you really get to see what else is on the island! We got back around 6, went and cleaned up and were ready for dinner by 7ish! I would recommend that idea to anyone, you won't regret it! Have fun and good luck!
Oh, and we signed up the morning before and had no problems and I just wore my swimsuit the whole time and took a water camera. I don't think you need to bring shorts, maybe a towel. I'm not sure why you would wear shorts to walk in the ocean and get into a boat, then they'd just get all wet and you'd have on wet shorts when you got off the boat. Right?
We did this with our 16 guests the day after our wedding, because we were married at sunset on 10/2/09. When I checked into the resort I booked the catamaran because all of my guests told me they wanted to go in advance. WE HAD A BALL! It was truly the highlight of the trip as a group...besides the wedding!