Shrimp and Scallop Ceviche

Name: Shrimp and Scallop Ceviche
Ingredients: 1/4 cup kosher salt, plus more for seasoning; 1 pound medium shrimp, peeled and deveined; 1/2 pound bay scallops; 2 lemons, juiced; 2 limes, juiced; 2 oranges, juiced; 1 cup peeled, seeded and diced cucumber; 1/2 cup finely chopped red onion; 2 serrano chiles, seeded and finely chopped; 1 cup seeded and diced tomatoes; 1 Hass avocado, peeled, seeded, and chopped into 1/2-inch pieces; 1 tablespoon roughly chopped fresh cilantro leaves, plus leaves for garnish; 1/4 cup extra-virgin olive oil