We visited Ocho Rios by accident this past October 2002 due to an itinerary change (hurricanes). Dunn's River Falls is pretty much what you've read here...but you can make it more bearable and enjoyable by taking your own taxi just off the docks (standard fee) and doing it yourself. Note the entrance landmarks when you enter and head for them when you leave instead of the shops on the right...which is where they'll direct you. That was the only sour experience we had with the park. We did the falls once with a group and once without and enjoyed it.
TIP: Bring cheap beach shoes or you'll have to rent them for $5...you can't do it barefoot.
TIP: Unless you pack extremely light and extremely tight (waterproof), you'll want to rent a locker at the TOP before going down. That will save you a hike.
TIP: Take waterproof camera photos, but be prepared to "hire" someone to do video. You can get another tourist to video you from the side about half way up because there's easy in/out access from a landing.
TIP: If a young couple asks you if the area is man-made or real, don't laugh (it happened to us)! It is so naturally clean that it looks Disneyish.
As for snorkeling, a couple we met basically walked around the beach inlet to the left of the pier (avoid going too much through the street...you'll get hassled). They asked and found some locals that took them out nearby in a large johnboat for much less than an excursion ($20?)..we wish we had thought of that!