Yeah we also liked Roatan much more than Negril. It seemed all we saw was sea grass.
Have to admit that we've never been to Roatan, but I would take exception to your characterization of diving in Negril.
I was looking through my pictures from our trip this past December for some Sea Grass. Closest I could get to a green background was this Flying Gurnard nestled in a patch of green.
This picture is a much better representation of the structure you'll be diving on.
We've been going there since 2003, and it just keeps getting more and more healthy, with much larger counts and more variety in the fish populations.