I have been diving in Jamaica on and off since 1995 and the diving is every bit as good as Cayman, Turks, and the other locations. There is the same sea life to be seen in Jamaica: spectacular corals, sponges, spotted eagle rays, sting rays, sea turtles, snapper, colorful fish... the whole shebang.
One piece of advice... Don't do dives right off the coast of Montego Bay or Ocho Rios... erosion caused by rain, hotel & city drains, and general fishing and tourist damaged have ruined the reefs. To see the real Jamaica you need to get out of the cities. Try diving near Falmouth, Runaway Bay, remote sites near Negril, the south coast or Port Antonio. GET OFF THE BEATEN PATH.
Frankly, I wouldn't go diving in a bay off any major city in the US, Australia, etc either! I mean, what would YOU expect to see off the coast of Miami, for example?
Great diving can be had in Jamaica, it isn't touted as a "premier" diving destination, which just means less crowded boats for us and less crowded dives in general, but stay away from the big cities.