Arrange fish fillets in a large baking dish. Sprinkle evenly on both sides with sea salt. Pour cold water to cover over fish. Chill 1 hour.
Whisk together 2/3 cup cold water and next 4 ingredients in a large bowl until mixture is smooth.
Drain fish, and pat dry between layers of paper towels. Dip fish in batter, allowing excess to drip off.
Pour oil to depth of 1 1/4 inches into a Dutch oven; heat to 370°. Fry fish, in batches, 10 minutes or until golden brown. Drain on paper towels. Serve immediately. Garnish, if desired.