Scotts Head Marine Reserve is an amazing place for snorkeling, diving and free diving. It is about 45 minutes drive from Cocoa Cottage, at the South West tip of Dominica.
There is a pristine reef just at the tip of the peninsula of Scotts Head, with a beautiful underwater wall. Huge sponges and coral line the wall and form one of the best snorkeling spots in the world. You can park your car on the peninsula, walk on the north-facing beach of the peninsula until you reach a small sand beach. Here you can enter the water and snorkel all the way up to the tip of Scott’s Head. The first 200 meters are not that interesting, but then it is really beautiful. You need to swim around the tip, almost until you reach the beach on the Atlantic side. You need to be a really good swimmer to snorkel at this spot! Take fins, don’t go alone, and only go in calm weather. When you reach the tip where the Caribbean Ocean meets the Atlantic, the current is getting strong, so be careful.
This link contains beautiful photos, some of them showing the reef with the wall: https://www.uncommoncaribbean.com/dominica/scotts-head-dominica/
There are some very famous dive spots at Scotts head as well. Arrange a tour with one of the local dive shops, for example with Dive Dominica. Another activity you can pursue here is Free Diving. The steep underwater walls and the crystal clear water are just perfect for diving into the deep.