Becky - how many in your group? This is important as some of the smaller operators provide a much more personalised service, but their boats can only accommodate smaller numbers.
I have stayed at Fort Young in Roseau, it is comfortable however I would not stay there again as the dive shop is not on site, and I found the hotel to be more suited to business travellers. (I would recommend though as the breakfast buffet was lovely!).
The dive shops that I used whilst in Dominica were Dive Dominica and Cabrits Dive.
Dive Dominica has large catamaran style boats, the crew are friendly and accommodating, but the service is impersonal and the dive sites are determined by the day of the week (or so it seemed). I preferred the diving in the South personally, but don't believe the hype about Champagne Reef, it's OK, but no where near as spectacular as I was lead to believe.
I have heard really good things about Nature Island, they are photographers themselves, and know what it is you expect, so if I may, I would suggest touching base with them to see if they can assist you.
The scenary in Dominica is stunning, so make sure that you fit in some time for land tours, and pack your hiking boots as the trials are worth it!
Hope that you enjoy your visit to Dominica.