Make Batter - In a large bowl, whisk together all the dry ingredients. Add in sparkling water and whisk to combine.
Make White Sauce - In a small bowl, mix together the mayonnaise, sour cream, and milk. Season with salt and pepper, juice from ½ lime, and a few dashes of hot sauce. Whisk to thoroughly combine.
Heat oil - Pour oil in high-sided skillet and heat to 375°F (190°C). Turn oven to 200°F (120°C).
Prepare Fish - Slice fish filets into pieces about 3-inches long and ½-inch wide. When oil is hot enough, add butter then dip a piece of fish in the batter to cover. Remove with tongs and gently lay into oil, careful not to crowd. Cook 3-4 minutes on one side, flip, and cook 1 minute more. Drain on paper towels and put on a wire rack over a baking sheet and move into warmed oven. Then repeat process until done.
To Serve - Put tortillas between a wet paper towel and microwave for 30 seconds. Put in a towel-topped basket and let guests assemble tacos as they like by laying 2-3 fish pieces in a tortilla and topping with a choice of shredded cabbage, white sauce, lime wedges, salsa, and hot sauce. To make a full meal, serve with refried beans and Mexican rice.