We went on a whale watching trip with Dive Dominica. There are a couple of operators on the island, and we chose Dive Dominica because they had a better web site and we could book by email. They are located about a 10 minute drive south of Roseau. The boat was a big catamaran. The whole trip was about 4 hours, but included a quick run to Roseau to pick up some passengers from the cruise ship. We saw 11 sperm whales.
The operator was very respectful of the whales, and never chased them. We didn't see any dolphins on our trip. The sea was a little rough, but not bad enough that a couple of Dramamine (motion sickness tablets) didn't take care of it. It was certainly nothing like the ferry ride over from Martinique. There are toilets on board and it would be a great trip with kids.