In a resealable plastic bag, combine 3 tablespoons lime juice, water, 1 tablespoons cilantro, garlic, cumin and cayenne. Add fish, seal bag and turn to coat; refrigerate, turning once, 1 hour.
In a bowl, combine vinegar, oil, 2 teaspoons honey, 1/4 teaspoons salt and 1/4 teaspoons pepper. Add cabbage; toss well.
In another bowl, combine sour cream, 1 tablespoons lime juice, zest, 3/4 teaspoons honey and 1/2 teaspoons salt.
Heat grill. Remove fish from marinade; season with remaining 1/2 teaspoons salt and remaining 1/4 teaspoons pepper. Grill, turning once, until just cooked through and lightly charred, 4 minutes per side.
Transfer fish to a cutting board and coarsely chop.
Grill tortillas, turning once, 30 seconds per side. Divide fish, slaw and avocado among tortillas, then top with sour cream mixture. Garnish with lime juice and extra cilantro, if desired.