The short answer
Grilled fish. Essaouira is a working Atlantic port, and sardines and other fish are split and grilled over charcoal at the harbour. On our day trip the harbour lunch of grilled fish and seafood straight off the blue boats is an optional add-on. On the way, the argan cooperative stop includes an amlou tasting.
In more detail
The drive west from Marrakech takes about 2 hours 45 minutes, 190 km, across the argan plains. The first food stop comes on the road: a women's argan cooperative, with argan pressing, a taste of amlou, the almond and argan spread, and the goats that climb the trees.
In town, the harbour is the lunch. Blue fishing boats unload in the morning, and the grills by the port cook what came in: sardines, other fish and seafood, split and grilled over charcoal. On our trip the harbour lunch is an optional add-on, so you choose on the day; bring cash for it.
After lunch you have free time in the walled medina, a UNESCO site of whitewashed alleys, spice and woodwork souks and Gnaoua music shops, and a walk on the Skala sea ramparts above the Atlantic spray.
Essaouira is breezy all year, so bring a windproof layer even in summer. The fish is the reason to go hungry.