Heat grill to medium-high. Season steak with ½ tsp each salt and pepper and grill to desired doneness (6 to 8 minutes per side for medium-rare). Transfer to a cutting board and let rest before slicing.
Meanwhile, in a bowl, whisk together lemon juice, oil, honey, and a pinch each salt and pepper. Fold in onion and tomatoes.
Cut watermelon into ½-inch-thick triangles and cut off rinds. Brush lightly with oil, then grill until lightly charred, 1 to 2 minutes per side. Divide among 4 plates.
Fold herbs into tomato mixture, then gently toss with arugula. Spoon on top of watermelon and serve with steak.