The short answer
Marrakech is inland, but fish is easy to find: N°14 Krita on Jemaa el-Fnaa has fried squid, shrimp and sole since 1966, a fried-fish eatery sits near the Ali Ben Youssef Mosque, and Latitude 31 in Bab Doukkala serves a seafood pastilla. For fish straight off the boats, a day trip to Essaouira on the Atlantic is the move.
In more detail
On the square, fried fish is one of the night-market staples alongside the grills. N°14 Krita, stall 14, has fried squid, shrimp and sole on the menu and has been cooking since 1966. In the medina, a small fried-fish eatery near the Ali Ben Youssef Mosque does the same away from the crowds.
For a sit-down meal, Latitude 31 in Bab Doukkala serves a Moroccan-Mediterranean menu including seafood pastilla, in an enclosed garden courtyard. Le Grand Bazar, a century-old hotel turned restaurant on Jemaa el-Fnaa, lists seafood among its Moroccan and French cooking, with a big rooftop facing the Koutoubia.
The real seafood town is Essaouira, about 2 hours 45 minutes (190 km) west. Sardines are split and grilled over charcoal at the port, and on our Essaouira day trip the optional harbour lunch is grilled fish and seafood straight off the blue boats. You also get the Skala ramparts and the walled medina.
At any stall, the fish should be cooked hot in front of you. Busy stalls turn their stock over fastest.