I know you said you already found a company, but I thought I'd send you some info. anyways. I was married in Ocho Rios in 2008 through Tropical Weddings Jamaica. We also used a cruise as the reception.
After the small mini reception at our wedding site. Basically snacks to hold everyone over. We booked a booze cruise for our dinner reception. It was very casual. We all changed into casual clothes before we left the wedding site and got on the bus. Good thing because the waters were a little rough and drinks were spilling. Also, the bathrooms are not the roomiest.
We booked through
We had a private "Taste of Jamiaca Cruise". You can find the info on their web-site. It started at 5 and lasted until 8. Basically, the boat staff did whatever we asked. They were very nice and friendly!! All my guests loved them. Very relaxed atmosphere! There was plenty of food and drink which was good I hear. I never had anything to eat, but can tell you the drinks were awesome!!!. We danced and saw the coastline with all the hotels. However, it gets dark quite early in Jamaica so about half of the cruise was in the dark. It cost $70 per person. After the cruise we were provided transportation back to our resort where we all went to the hotel buffet/snack bar for more food. Then we hit the resort Disco. Also, you may want to bring a couple of CD's with music for dancing. A lot of my guests did not dance for a long time. I would suggest bringing some of your own CDs of music your guest will dance to. At first, all that was played was Carribean. Fun music, but not stuff that everyone would dance to. Once they started playing North American music then my guests felt more comfortable dancing.
This company takes off from Ocho Rios, Mahogany Beach. We got a bus ride to Mahogany Beach. which is about 5 mins from Sandals Grande Ocho Rios. Before we went on the boat, we all had to sign a waiver. Once that was completed, we were off. I also asked my guests to bring extra cash for tips, seeing the staff did not work at a resort.
Hope this helps. If you need anything, just PM me.