The salsa combines sweetness from watermelon; acid from tomatoes and lime; a bit of heat from the chile; and a pop of herbal freshness from cilantro and mint. It pairs particularly well with red snapper and would be lovely with most any mild white fish. To get the fish skin good and crisp, be sure to forgo a nonstick skillet in favor of a stainless-steel or cast-iron one; reach for a 12-inch skillet (or larger) so the fish doesn't get overcrowded—or cook in 2 batches if using a smaller pan. Serve with grilled fresh corn, sliced avocado, and a margarita for an easy, fast, elegant dinner.
Stir together tomatoes, watermelon, red onion, chile, cilantro, mint, lime juice, 1 tablespoon canola oil, and 1/4 teaspoon salt in a medium bowl. Set aside.
Pat fish dry with paper towels. Cut 3 shallow slits into the skin side of each fillet. Sprinkle fillets with pepper and remaining 1/2 teaspoon salt. Heat remaining 2 tablespoons oil in a large stainless-steel or cast-iron skillet over high. Add fillets, skin side down, and cook until skin is browned and crisp, about 5 minutes. Turn fillets; cook until lightly browned and done, 1 to 2 minutes. Serve fish immediately, skin side up, topped with salsa.