Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Place fish in a large resealable plastic bag set in a shallow dish. Set aside. If using cumin seeds, in a small dry skillet, heat cumin seeds over medium heat for 1 to 2 minutes or until aromatic, shaking skillet occasionally. Crush cumin seeds in a mortar and pestle.
In a small bowl, combine crushed cumin seeds or ground cumin, the lime peel, lime juice, oil, 1/8 teaspoon of the crushed red pepper, and the salt. Pour over fish in bag; turn to coat fish. Seal bag. Marinate in the refrigerator for 30 to 60 minutes, turning bag occasionally.
Meanwhile, for salsa:
In a small bowl, combine watermelon, sweet pepper, green onion, mint, and the remaining 1/8 teaspoon crushed red pepper. Set aside.
Drain fish, discarding marinade. For a charcoal grill, grill fish on the greased rack of an uncovered grill directly over medium coals for 6 to 10 minutes or until fish flakes easily when tested with a fork, gently turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place fish on greased grill rack over heat. Cover and grill as above.) Serve fish topped with watermelon mixture. If desired, serve with lime wedges. Makes 2 servings (1 tuna steak and 2/3 cup salsa each).