La Marsa has the best concentration of restaurants in greater Tunis outside the central medina, and it is markedly more cosmopolitan than the rest of the city. French bistros, Italian trattorias, Lebanese restaurants, Asian places, and good Tunisian seafood are all here, mostly clustered around the corniche and the area inland from it.
For fish, Le Golfe and Au Bon Vieux Temps are reliable, set on terraces with sea views. For a Tunisian meal, try Restaurant le Saf-Saf (a different establishment from the café) or one of the small couscous places in the old town behind the corniche. The bakeries are particularly good — La Marsa's French-style patisseries supply much of Tunis. The town also has a small but lively bar and café scene in the evenings, less restricted than in the rest of Tunisia.