Heat 1 teaspoon oil in a large nonstick skillet over medium-high heat. Sprinkle fish evenly with 1/8 teaspoon salt and 1/8 teaspoon pepper. Add fish to pan; cook 4 minutes or until browned. Carefully turn fish over; cook 3 minutes or until fish flakes easily when tested with a fork or desired degree of doneness. Remove fish from pan; keep warm.
Combine watercress, lettuce, tomatoes, and shallots in a large bowl. Combine vinegars, honey, mustard, remaining 1/8 teaspoon salt, and remaining 1/8 teaspoon pepper in a small bowl; gradually add remaining 1 1/2 teaspoons oil, stirring with a whisk. Pour vinaigrette over lettuce mixture; toss gently to coat. Arrange 2 1/2 cups lettuce mixture on each of 2 plates; top each serving with 1 fillet.
The vinaigrette was wonderful and the choices of lettuce were a perfect accompaniment to the mild flavor of the mahi mahi. I'm not a huge fan of tomatoes and will reduce the amount for next time but definitely a dish worth trying!