I have never dived Montserrat, but I have dived a lot of the islands around Montserrat so I feel vaguely qualified to give an opinion.
Caribbean diving tends to be warm water, good viz, low currents. Marine life varies, but as a rule of thumb, the poorer the country the fewer the fish (because more people eat fish on a subsistance basis: Cayman, BVI - lots of fish; St Lucia, DR, Jamaica - bugger all fish).
Montserrat is not known as a dive location, but I wouldn't read too much into that. It is not an easy place to get to, and the active volcano means it is pretty hard to promote as a tourist destination. Plus there are a lot of places offering excellent Caribbean diving that are not very well know outside of the Caribbean (Martinique has fantastic wreck diving, but nobody really knows. St Vincent has excellent reef diving, but you never hear about it). There are certainly commercial dive ops in Montserrat.
Hope that helps. I was going to take my son there on holiday a couple of years ago, but I gave up because, frankly, it is a bit of a pig to get to. You have to connect via Antigua, which itself is a pain.
If you do go, be sure to post a trip report.