1. To make the Quick Pickled Vegetables: Combine vinegar, sugar and salt in a saucepan with the water; bring to a boil over high heat and stir to dissolve sugar. Add carrots, onions, and jalapeños, and reduce heat to medium-high. Simmer until carrots are just tender, 10 to 12 minutes. Pour into a bowl; let cool completely before covering and refrigerating. (Pickles will keep up to 2 weeks. Recipe makes 1 quart.)
2. Stir together cumin, oregano, chili powder, and 2 teaspoons salt. Mix in the olive oil and cilantro. Use a sharp knife to make shallow slits about 1½ inches apart into the fish skin; transfer to a large dish and rub both sides with spice mixture. Refrigerate, covered, for 30 minutes to 2 hours.
3. Toss together cabbage and 1½ teaspoons salt in a medium bowl. Let sit 30 minutes.
4. Heat grill (or grill pan) to high (if using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 2 to 3 seconds); lightly oil grates. Place fish, skin side up, on grill. Cook, without moving fish, until opaque throughout, 3 to 5 minutes. Carefully flip, using two spatulas if necessary. Cook until fish flakes slightly when pressed in the center, 5 to 7 minutes more.
5. Transfer fish to a serving platter; let cool slightly, then shred with a fork into bite-size pieces. Garnish platter with lime wedges and cilantro sprigs. Serve warm or at room temperature (within 1 hour). Allow guests to assemble their own tacos, layering fish with cabbage, avocado, sour cream, hot sauce, pickled vegetables, and other toppings, as desired.