This is, by far, the best place to eat in Essaouira. All the locals told us to come and they hit the nail square on the head. I'd suggest skipping the grilled fish stalls near the port (tourists-only) and heading here for some equally-delicious grub at a fraction of the price.
You can enter and buy your own fish, but they sometimes try to push lesser-selling fishing or, on the other end of the spectrum, really expensive ones like jumbo prawns, but just check out what others are buying and always make sure to get the price before they gut and scale them. We got octopus, cuttlefish, prawns, and snapper for two for a total of 14 dollars. In other words, a steal. You can also go straight to the restaurant and tell one of the young men there what you want and he'll get it for you. That way, we got a sea bass and a plate of salty, smoky sardines for 8 dollars.