About 20 minutes south of Roseau (an easy drive), there is a park called "Champagne". It's actually just a small strip of land and boardwalk bordering the ocean. The attraction here is that there is geothermal activity under the ocean floor which causes bubbles to float up. The bubbles are not really visible from the top of the water - you have to snorkel to experience it. It's a very pretty effect, although the bubbles aren't so strong that you can really feel it. It's not like being in a hot tub. The snorkeling itself wasn't that impressive - some fish but the coral was not in good shape. Apparently if we'd swum further out to the reef, we would have seen more, but we were there alone and didn't feel comfortable doing that. It is apparently one of the top dive spots in the world.
The park itself is very well set up - there is lots of parking at the side of the road, and a little dive shop / snack restaurant which will rent diving equipment, and also snorkeling equipment. There is even a shower we could use - which ironically had hotter water than our hotel. There is a clean bathroom as well. There is a small entrance fee to the park ($3 US).