On the recommendation of my friend's mother, who is from Marrakesh, we followed our noses until we were led to Samak al-Bahriya. The outdoor patio was jam-packed with (mostly) Marrakeshi and a few tourists and their Lonely Planet guidebooks. I couldn't quite figure out the system to get a table, but with patience, you will be seated eventually.
We ordered four fish platters that come with all types of seafood, including shrimp, calamari, and two types of white fish—all simply breaded and with a wedge of lemon on the side. Each order also comes with fragrant yellow rice, salad, olives, and bread. It's a fun place to sit outside, enjoy some unexpected fresh fish—in the desert, of all places!—and watch the busy happenings going on in the street.
Al-Bahriya is located on Boulevard Moulay Rachid not too far from the train station (but if you find yourself lost, you can always follow your nose).