You can do some basic zip lining at YS Falls or Jam West Speedway. Nothing too elaborate, but good enough for a introduction if you've never done it before. I once did a full zip line course up near Ocho Rios, but that is a few hours away from Negril.
You won't find any jet skis in or around Negril (to the relief of many Negril Boardies

). Instead, hire a glass bottom boat to take you snorkeling on the reef or in the caves of the West End.
ETA: Livin beat me to it, with basically the same info! I took too long to click "Post".