For the sauce: Pour water and vinegar into a saucepan, and bring to a boil over high heat. Stir in sugar, ginger, garlic, red pepper flakes, ketchup and cornstarch; simmer for 5 minutes or until it starts to thicken. Pour into a bowl and refrigerate until ready to serve.
In a bowl, mix together the flour, baking powder, and pinch of salt. Slowly whisk in the seltzer until the batter is thick and free of any lumps.
In a pot heat the oil on medium heat. (Make sure the oil is hot)
Coat the shrimp in cornstarch and then dip into the batter (allowing the excess to drip off) and then slowly immerse the battered shrimp into the hot oil. Work in batches if needed as to not overcrowd the oil.
Cook for 2 - 4 minutes and gently turn the pieces over and cook until golden brown, about another 2 - 4 minutes. Drain on a paper towel lined plate and serve with the sauce.