Preheat the oven to 200 degrees . In a large saucepan, cover the potatoes with 2 inches of water. Add the Old Bay; boil over medium-high heat until tender, about 15 minutes. Drain; toss with the butter.
Meanwhile, season the cod and shrimp with 1/2 tsp. salt and 1/4 tsp. pepper. On a large plate, mix the flour, garlic powder and cayenne. In a medium bowl, whisk the egg and egg white. On another large plate, mix the breadcrumbs and cornmeal.
Working with one fillet at a time, dredge the cod in the flour mixture; shake off excess. Coat with the egg mixture; shake off excess. Transfer to the breadcrumb mixture; press to adhere. Place on a baking sheet. Repeat with the shrimp.
In a large nonstick skillet, heat the oil over medium-high. Cook the cod until opaque in center, 3 to 5 minutes per side. Transfer to a rack set in a baking sheet; place in the oven. Cook the shrimp in the skillet until golden, 1 to 1 1/2 minutes per side. Serve with the cod, potatoes and tartar sauce.
In a small bowl, mix canola mayo, yogurt, gherkins, shallot and capers.