Combine 2 quarts water and 1/4 cup salt in a large saucepan; bring to a boil over high heat. Add shrimp and immediately turn off the heat. Let the shrimp sit until just cooked through, about 2-3 minutes. Transfer to a cutting board until cool enough to handle, about 10 minutes. Cut shrimp in half.
In a food processor, blend together garlic, shallot, jalapeno and zest. Add lime juice, grapefruit juice, salt, sugar and blend. Set aside 3 tablespoons marinade for dressing. Place shrimp in a bowl and cover with citrus marinade. Cover bowl with plastic wrap. Refrigerate for at least 30 minutes and up to 2 days.
Puree remaining 3 tablespoons marinade with 6 tablespoons grapeseed oil. Season with salt to taste. Pour into a squeeze bottle.
In an Asian flat bottom spoon or endive spear place a grapefruit segment, an avocado slice and a piece of shrimp. Drizzle with a little dressing. Garnish with cilantro.