Whisk together first 6 ingredients; pour into a large heavy-duty zip-top plastic bag, and add mahi mahi. Seal and chill 8 hours, turning occasionally.
Combine sesame seeds and flour in a shallow dish. Remove fish from marinade; pat dry with paper towels. Coat fish on all sides with sesame seed mixture.
Heat sesame and peanut oils in a large nonstick skillet over medium-low heat. Add fish; cook 2 to 3 minutes or until golden. Turn over, reduce heat to low, and cook 2 to 3 minutes until fish flakes easily with a fork. Serve over Cucumber-Watercress Salad.