Make the cabbage slaw: Combine the cabbage, onion, tomato, cilantro, jalapeño, lime juice, and salt in a large bowl. Toss well, then set aside.
Make the crispy fish: In a large bowl, combine the flour, Old Bay, and baking powder. Add the beer and whisk until smooth. Let sit for 15 minutes.
Heat the vegetable oil in a large pot until it reaches 350°F (180°C).
Coat the fish in the batter, then transfer to the oil and fry until golden on the outside and cooked through, 5-7 minutes. Drain the fish on a wire rack set over a baking sheet lined with paper towels.
Make the avocado crema: Add the avocados, sour cream, lime juice, and salt to a blender and blend until smooth.
To assemble the tacos, add a bit of the cabbage slaw to a warmed tortilla. Top with a piece of fried fish, avocado crema, and sliced radish.