There's a really nice wall all along the left side of the cove. Lots of crevaces etc. Not sure how much marine life you'll see this time of year, probably just anemones. Those are usually further out and a little deeper. In the summer you can see all kinds of things - skates, crabs, lobster, sea ravens, all kinds of local fish, etc... As you head out along the wall, the bottom drops down to about 50-60ft out by the point and around it.
The middle of the cove is pretty much all sand. Not that much to see there. A few boulders strewn about, and that's about it. Some people have seen torpedo rays in the cove, but once again, that was in the summer.
The right side of the cove is much shallower, you'll be lucky to hit much more than 30. It's very rocky - the rock formations you'll see on the surface continue underwater. It is still a very nice dive there. Once again, lots of crevaces, caves, etc. It's not a wall, but is still a very nice dive. Lots of lobster there in the summer. When I dive Folly, I'll usually dive the wall on the 1st dive and the boulders on the 2nd. The middle of the cove isn't really worth the effort, as far as I'm concerned. There isn't much there, especially during the day. On summer nights, though, the crabs and the lobster come out of their holes and start marching all over the cove, including the middle.
The entry in the cove is fairly easy, but not that good for your feet - lots of little round slippery boulders and stones. Make sure you have thick soles on your boots, otherwise watch where you step.
Hope this helps.
-Roman.