Just got back from Negril last week. Had a blast. Stayed at the Rockhouse the first few days for a friend's wedding, then went to Couples all-inclusive for the 2nd half of the week.
A great dive shop is Sun Divers. I cannot remember the dive leader's name - did write it down in my log book though.
Dives:
Deep Plane - 90'
Shallow Plane & Arches - ~45'
Throne Room - 50'
These are the top 3 sites, there is one more, but I do not remember the name - I did not do it. My wife is only OW cert, so I did the Deep Plane without her. Saw a 7' nurse shark, a few dolphins, a nice sized ray and a turtle over the course of 6 dives.
Things on top:
Rick's Cafe - cliff jumping from about 45' up - kind of flashy/showy - very famous for the playboy bunnies and rolling stones hang-out in the 70's/early 80's.
Pirate's Cave - cliff jumping from about 42' - way more low-key than Rick's, quaint place. The owner has some GREAT old newspaper clippings if you ask about them.
A few good restaurants. My wife had a favorite, but I cannot remember the name - nice ambiance, walk along the 7-mile beach afterwards.
If you are going solo, I would hit Sun Divers right away and get in with them. Young crowd, may be able to give ideas/hangout, etc.
Have fun! My wife and I had a blast! It was her first non-cert dives and she loved the 82-84 degree water.