The key to the texture of these crispy-chewy seafood pancakes is in the specialty Korean pancake and frying mixes. See the Note for details on where to stock up. Slideshow: More Korean Recipes
Make the dipping sauce: In a small bowl, stir together soy sauce, vinegar, and sugar.
Make the pancake: In a large bowl, whisk together pancake mix, frying mix, and salt. Whisk in 1/2 cup water and egg. Fold in scallions, oysters, clams, shrimp, and squid.
Heat sesame oil in a 10-inch skillet over moderately high heat. Quickly add batter to pan and spread to edges with a rubber spatula while shaking pan. Cook until bottom is browned, about 5 minutes, shaking pan occasionally to keep it from sticking. Flip and cook until batter is set and bottom is browned, 4 to 5 minutes. Transfer to a serving plate.
Using scissors, cut pancake into 4 or 8 wedges and serve with the dipping sauce.