Champagne Bay in the town of Soufriere.  You can get there by boat or by taxi.  Taxi service from Fort Young Roseau is $40US round trip.  You decide the amount of time to spend there and let your driver know when to pick you up.  2 hours is recommended.  Entry is $2, there is a snack bar and you can rent snorkel gear here.  Walk down the boardwalk to the end.   If you have towels and clothing to leave, put them into a ledge in the hill in case of rain.  Bring zip lock bags for anything that must stay in your possession.  There is a rubber mat that allows you to slide on top of the rocky beach into the ocean.  A local guide suggests going straight out to the white buoy, then hang a left to the red buoy, then head toward shore and return along the rocks.  That route is nice, but if you want to see the actual champagne bubbles, go straight left  from the rubber mat entry.  The bubbles are in a relatively small area of Champagne bay and are just off the rocks to the left of the mat entry.  You will see a lot of fish variety and coral.  Parrot fish, trunk fish, spotted moray eel, sea snake, squirrel fish, butterfly, angel fish...   The water is relatively warm and a wet suit is not needed.  2 hours in the water should be comfortable.  This is an awesome place not to be missed by even the most experienced diver/snorkeler.