Preheat the oven to 450 degrees . Rinse the potatoes; pat dry with paper towels. Toss with EVOO and Old Bay; arrange in a single layer on a foil-lined baking sheet. Bake until golden and crisp, 20 minutes.
Meanwhile, in a medium bowl, whisk the seltzer, flour and salt until nearly smooth. In a large heavy skillet, heat 1/2 inch canola oil over medium. Pat the fish dry with paper towels. Working with one fillet at a time, dip the fish into the batter, coating both sides, then carefully place in the skillet. Cook the fish, turning once, until batter is puffy and light golden, 6 to 8 minutes. Drain on a paper towel-lined plate. Serve with the fries and tartar sauce.