Being a professional beach bum, I can tell you that Praia do Forte's not ideal for swimming, as the sea bed is stony and craggy. The beach next door, Itacimirim about 7km away, is far better for that.
Praia do Forte is good for diving and the village itself is pleasantly organised. There's a big choice of places to stay, to go shopping and to eat.
I often visit family at Praia do Forte and my favorite attractions are the Projeto Tamar that protects sea turtles (I go every time I'm there and tickets are around 6 USD) and the Projeto Baleia Jubarte, a right whale conservation project. You can also go on whale watching tours between July and September. At one time you could go out on a twin-engine, but I'm not sure if that's still possible.
The north-eastern beaches are not to be missed during a trip to Brazil . If whale watching is on your itinerary, then Praia do Forte is easier to access than the protected Abrolhos Archipelago, which is 1hr by plane from Salvador, followed by 2hrs on a bus and 5hrs by boat!
After touring the cultural sights at Salvador, Praia do Forte is a great place to relax and enjoy Bahia's coast. A one time fishing village, Praia do Forte has since become a favorite beach resort for Salvador's residents. The main, pedestrianized street is lined with shops and restaurants. And the 15 kilometer long beach has a perfect mix of fine sand, coconut trees and a warm sea. It only takes a few minutes walk from the main beach to reach a heavenly surrounding countryside. It provides a refreshing contrast with the touristy center. At low tide, the coral reef forms natural swimming pools - take a plunge with black, yellow or blue stripey tropical fish!
There are two interesting ecological projects at Praia do Forte that are worth a mention. The first is the Projeto Tamar, which is a marine turtle conservation project. Their center isn't big, but it's a great way to see a wide variety of species and specimens. Between December and February they release hatchlings onto the beach. I happened to be there at the time and it was a really sweet moment. The second project is run by the Institut Baleia Jubarte, who organize marine mammal watching boat trips. Not the best thing to do if you get sea sick, as it can sometimes be a rough ride.