Ralph Rubio is widely credited with popularizing the Baja-style fish taco in the early 1980s (via San Diego Magazine). Baja, a fishing town in San Diego, was known for its beer-battered tacos, complete with corn tortilla, cabbage, and Mexican crema.
Modern fish tacos emerged in the 1950s in the Baja city of either Ensenada or San Felipe; it's an ongoing debate, with both cities claiming to be the "home" of the fish taco. From their tiny stands, street vendors in these cities produced simple, inexpensive fare fast.