The fish taco is San Diego's signature dish, and the gap between a good one and a bad one is large enough to matter. The basic format is beer-battered white fish (typically cod or mahi-mahi), fried until crisp, served in two corn tortillas with shredded cabbage, white crema or mayonnaise, pico de gallo, and a squeeze of lime. Variations exist — grilled fish, baja-style with salsa verde, Ensenada-style with chipotle — but the battered and fried version is the reference.
The best fish tacos in San Diego are mostly found at small taquerias in Barrio Logan, City Heights, and along the beach communities. Oscar's Mexican Seafood (multiple locations) is widely considered a benchmark. Rolf's Mexican Food and Mariscos German in City Heights are less known to visitors and better for it. The Rubio's chain that popularised the dish has expanded to mediocrity; seek out the independent spots.
Pacific Beach and Ocean Beach have concentrations of taco shops within walking distance of the water. A fish taco and a beer after a morning in the ocean is a legitimate San Diego ritual.