The San Diego fish taco is defined by its white sauce: a thin, tangy crema, usually a mayonnaise-and-citrus or yogurt blend, drizzled over fried fish in a warm corn tortilla. That sauce is the whole argument. Fried fish in a tortilla without it is dry and one-note; the white sauce supplies the fat and the acid in a single stroke, clinging to the craggy batter and binding the fish, the cabbage, and the tortilla into one coherent bite. It is the element that makes this a specific regional sandwich rather than a generic fried-fish fold, and everything else in the build is arranged around delivering it.
The craft is keeping a crisp coating crisp inside a soft, wet wrap. A mild white fillet is cut into strips and fried in a light beer batter that has to stay crunchy against sauce and shredded cabbage, which is why the batter is kept thin and the fish goes in hot and comes out fast. The corn tortilla is warmed on a comal so it is pliable, and it is doubled in the standard build because a single corn round cannot carry fried fish and sauce without tearing. Shredded cabbage, not lettuce, is the deliberate crunch: it holds its texture under the warm fish and the white sauce where lettuce would wilt. A squeeze of lime and a spoon of salsa or pico are the sharp acidic counter that a fried, sauced core needs so it does not read as one heavy note. It is built and handed over in seconds at a beach-town stand, eaten standing up, structure intact only as long as it is fresh.
The variations are a settled grammar rather than free improvisation: battered and fried is the baseline, grilled fish a lighter reading that drops the crust, the white sauce moved between mayonnaise-based and crema-based, cabbage red or green. Each is a known order. It sits in the broad American taco, burrito, and wrap family, where a flexible bread folds a meal into one hand, and its close relatives, the Baja-style shrimp taco and the grilled-fish build, deserve their own articles rather than being crowded in here.