I have been to Dominica a couple of times, mostly business. But there are not a lot of options for lodging and dive ops. Most of the Hotels are in the same area and there isn't much difference between them, except for the size. The last time I stayed at the Fort Young Hotel. It was a nice hotel with a nice bar and a couple of restaurants, it also had a dive operation on site and close to town, theb rooms have nice balcony's that over look the ocean. There is really not much to do in the evenings in town so this hotel was the best when not diving. The whole Island is a rain forest, so you should spend time on some of the trips into the jungle, you won't be disappointed.
If you take a taxi from the airport to Roseau, don't sit in the front seat. Most of the way is a one lane road with lot's of twists and turns and it seems like a game of chicken when cars approach.