- Heat grill.
- Place watermelon on two sheets of paper towel to absorb excess moisture. Set aside.
- Sauce: Combine Greek yogurt with cucumber, watermelon rind, mint, cilantro, lemon juice, lemon zest, sugar, ginger and chilies. Salt to taste. Set aside.
- Spray fish with olive oil spray and sprinkle with salt and pepper. Grill for 3 to 5 minutes on each side or until fish is flaky and no longer transparent throughout.
- Top each serving of fish with 1/4 of the tzatziki, then sprinkle with 1/4 cup each of the watermelon.