In a large bowl, whisk together flour, cornstarch, baking powder, salt and cayenne. Whisk in 13 Tbsp. cool water and oil until blended, thick and foamy. Set batter aside.
Pour oil to about 1 1/2 inches deep in a large, deep skillet. Heat oil to 375°F. Meanwhile, break up any lumps of coconut and spread out on a baking sheet lined with wax paper. Pat shrimp dry with paper towels. Dip shrimp, one at a time, into batter to coat completely; shake off excess and place on top of coconut. Roll each shrimp in coconut to coat; shake off excess and transfer to a wire rack.
Slip shrimp into hot oil, 6 to 8 at a time, and cook for about 1 minute, or until golden brown and crisp; regulate heat to keep temperature at 375°F. Transfer shrimp with a mesh skimmer, slotted spoon or tongs to a paper towel-lined baking sheet to drain. Repeat with remaining shrimp, keeping oil as clear as possible by scooping out debris with skimmer or a small strainer. Transfer shrimp to a serving platter and serve hot with cocktail sauce on the side.