Toss tuna with soy sauce and vinegar; refrigerate, covered, 30 minutes. , Drain tuna, discarding marinade; pat dry. Sprinkle tuna with sesame seeds and pepper. In a large skillet, sear tuna until browned and center is medium rare or slightly pink; remove from skillet., On each of 16 appetizer skewers, thread 1 ginger slice and 1 tuna cube. If desired, place on a watercress-lined patter. Serve with wasabi mayonnaise.