Big Frank, I walked down to the fishing boats tied up along the river (by the bridge before the roundabout), and asked if anyone could give me a ride up the river to see the morass. It won't take long before you have a volunteer or two. When I went a couple of years ago, I think I paid about $40 US total for two of us. Let them know you want a slow ride to view the scenery, choose somebody that's got a cool vibe, and enjoy!
Of course in Negril, almost anything can be arranged, if you prefer to be picked up on the beach at your hotel that would be no problem.