Steaming the shrimp helps prevent overcooking and keeps it succulent. Watermelon makes a surprisingly good pairing.
Fit a steamer basket into a stockpot. Add 3 cups water and vinegar; bring to a boil. Add shrimp; cover and steam 5 minutes. Remove shrimp to a baking sheet; chill 15 minutes.
Stir together shallots, soy sauce, lime juice, and honey in a bowl. Let stand 10 minutes. Whisk in canola and sesame oil.
Add watermelon, lettuce, spinach, avocado, and 1/4 cup cilantro to shallot mixture; toss to combine. Place salad on a large platter, leaving any remaining dressing in bowl. Add shrimp; shichimi togarashi, if desired; and remaining 1/4 cup cilantro to bowl. Toss to combine. Place shrimp mixture on salad. Sprinkle with sesame seeds.