How to Make It
Using a cleaver, carefully remove pointed end of coconut. Drain coconut, reserving 1 cup coconut water; set aside. Carefully break or cut coconut into large pieces. Using your hand or a spoon, separate coconut flesh from shell; discard shell. Cut flesh into 1 1/2- by 1/8-inch strips to equal about 1/4 cup; set aside. Reserve remaining coconut flesh for another use.
Roughly chop onion to equal 1 tablespoon. Thinly slice remaining onion to equal 1/2 cup; set aside. Reserve any remaining onion for another use. Thinly slice 1 cucumber crosswise to equal about 1/4 cup; set aside. Roughly chop remaining cucumber and second cucumber.
Combine chopped cucumber, chopped onion, reserved 1 cup coconut water, tomatillos, lime juice, cilantro, chopped serrano, chiltepín chile, and salt in a blender. Process until smooth, about 20 seconds. Pour mixture through a fine wire-mesh strainer into a bowl; discard solids.
Stir together scallops, shrimp, and tomatillo mixture in a medium bowl. Let stand at room temperature until scallops are mostly opaque, about 10 minutes. Divide seafood mixture evenly among serving bowls (about 2/3 cup per bowl). Season with salt to taste. Top each serving evenly with reserved coconut slices, onion slices, and cucumber slices. Garnish with finger lime pulp or lime wedges, and serve with chips or tostadas.
Chiltepín chiles can be found at Latin markets or amazon.com.