Having spent many many happy hours snorkelling in Brewer's Bay, I am always a bit surprised to hear people tell me they are disappointed with the reefs and wildlife they have seen there. I wonder what has conspired to make their experiences bad ones and I always come back to the same two things: either their expectations are too high, or they don't know where to look.
Let's get one thing straight, Brewer's Bay is no Horseshoe Reef. Even though it's not that vibrant, Brewer's Bay does offer some of the best snorkelling from the beach available on Tortola. You have to be a pretty decent swimmer to enjoy it and you also have to know where to find the reef.
I have borrowed this screenshot from Google Maps to illustrate my point. In this satellite image, you can clearly see the reef areas -- they appear in lighter yellow and green colors. There's a big triangular area in the middle that extends up to towards Anderson Point, and then there's reef lining the wall on the south side of the bay.
The best coral I've seen is in the areas marked with the white lines in the image above. As you might note from the scale, these areas are over 1000 ft from the beach. Brewer's Bay is usually fairly calm, so anyone who feels reasonably comfortable in the water can probably swim out to these areas. Plenty of sea fans and elkhorn formations and the animals I've seen in these areas include sea turtles, lobster, porcupine fish, and plenty of large parrot fish. Additionally, the stingrays seems to prefer the sandy area between the two lines, about halfway between shore and where the map is marked "boating".
If you don't think you can make it out the 400 yards or so to these spots, or if you're not comfortable swimming and just want to stand in the sand and see some fish, you may find snorkelling at Brewer's a bit unrewarding. There are a few spots close to shore, about halfway between the two beach bars, where the reef comes in and you'll see a few rocks, maybe a small brain coral, and some gobi and other small fishes.
Hopefully, with the knowledge of the snorkelling spots in Brewer's Bay, you'll have a more satisfying day in the water.