I think I remember seeing this as an excursion in the book on the resort. Check with them when you arrive. I am sure if they dont have it listed as an excursion, they will find a way to get you there. That is just what they do. My friend just returned from home, although she stayed at a different resort, and she went on one, and said it was the highlight of their trip.
The only reference I saw to the private sunset catamaran cruise was for the Ocho Rios side of the island CSS and CTI. Not saying there isn't one, just have not seen it in 4 years, but wasn't looking either.
Not sure where to book it, but I'm sure the staff could tell you that and I'm sure you do it right at the resort. I believe the water staff take you out on a smaller catamaran and it costs $100 per couple. They bring drinks and munchies and take you places that the included and bigger catamaran does not take you. I've seen that people have been brought to other snorkeling places and someone even said they went further down the coast than the other catamaran. I think they just started offering this and it is all done thru the resort. It seems if you had resort credits you could use those to book this too.
I think we may try it the next time we go. It sounds fun, adventurous and romantic.
We did the private cruise when we were at CSA in July. It was $120 for two hours. They use a deep sea fishing boat with cushions on the front, where you sit. You book this through water sports. They brought champagne, beer, and juice. We also had some little sandwiches, fresh fruit, and cheese with crackers. We had a great time!
We have asked the company that does the sunset cat cruise at CN and CSA about a private cruise when we were onboard with them for the resort sponsored cruise. They said that they would do that, but I can't remember what they would charge.