Melt butter in a medium saucepan. Add lemon juice, soy sauce, mustard, dill sauce, crab and shrimp boil, and garlic powder; mix well. Cook mixture over medium heat, stirring occasionally, until thoroughly heated. Remove from heat; cool.
Rinse steaks thoroughly in cold water, and pat dry. Place steaks in a large shallow container; pour prepared marinade over steaks. Cover and refrigerate 4 to 6 hours, turning steaks occasionally.
Remove steaks from marinade, reserving marinade. Grill over hot coals 18 minutes on each side or until fish flakes easily when tested with a fork, basting frequently with reserved marinade. Transfer steaks to a warm serving platter; serve immediately with lemon slices.