To answer your question: Best to worst, Dominica tied Grenada, St Croix, St Kitts, St Thomas. All typical Eastern Carib, all tired reefs with whole species missing, large critters hard to find, generally fished hard for the last 150 years. (Dominca as an example, has lots of scrap mono-filament line, fish nets and active fish traps all over the Marine Park) All are good places to enjoy diving if you happen to be there, especially in winter.
I think this ranking is about right. Dominica is, perhaps, a bit less 'tired' because it is much less dived over than the other four because it has far fewer tourists. Almost no sandy beaches, very limited tourist infrastructure, two tiny airports, and hard to get to by air are all limiting factors that help to keep the small reef structures reasonably healthy. They are worked over by local fisherman, despite 'park' designations.
Conservation takes a back seat to economic imperatives on an island which, excepting Haiti, is the poorest in the Caribbean. Poorest in the Western Hemisphere, for that matter. The population of Dominica is tiny, less than 70,000. Everyone knows everybody else, and it would be hard to find a government minister, police officer, judge, or conservation official who is not related to some the fisherman who ignore restrictions. The curse of small islands; you don't give summonses or impose fines on your uncles and cousins.
An unstable, corrupt political establishment and the extreme vulnerability poverty creates are other elements in Dominica's catalog of problems, and the rapidly expanding Chinese presence does not bode well. Almost no assistance from the US. China, Cuba, and Venezuela are pretty much their only friends. I was there not that long ago when there was almost no gasoline to be had. Even busses and delivery trucks stopped running until an emergency supply came in from a Venezuelan tanker that had been rerouted.
It's still the loveliest most unspoiled island in the Caribbean, despite the 500 years of abuse that has been visited on the Caribbean Basin. The impact of tourism can best be gauged by observing what has happened to the reefs over time, by comparing the most heavily burdened by tourism with those least affected. Probably the most serious damage to the reefs in the southern end of Dominica has been inflicted by those wretched death stars, the mega cruise ships.
'Southern Cross' by CSN is one of those things that has stayed with me, in my mind for more decades than I like to think about.
