Place refried beans in small pan or pot with about 1/3 cup water and some hot sauce. Reheat beans and reduce heat to low to keep warm.
Combine cabbage with onions and dress with juice of 2 limes and some salt, then reserve or chill. Cut remaining lime into small wedges to pass at table for topping tacos.
Preheat oven to 250˚F and arrange a cooling rack over a baking sheet to keep fish crisp in oven while frying up batches.
Pour about 2 inches of fry oil into a large heavy pot such as a Dutch oven. Heat over medium to medium-high heat. When oil is ready for frying bubbles will surround and flow from a submerged wooden spoon handle.
Meanwhile, pat fish dry and season with Old Bay and salt.