Whisk the cake flour and rice flour together in a medium glass bowl and divide it in half. Set aside.
Heat the vegetable oil in a 5-quart Dutch oven over high heat until it reaches 375ºF on a deep-fry thermometer.
Once the temperature reaches 365ºF, whisk the egg, seltzer water, and vodka in a medium mixing bowl and divide it in half. Put half of the mixture in the refrigerator. Pour the other half of the liquid mixture into half of the dry mixture and whisk to combine, about 10 to 15 seconds. Some lumps may remain. Set the glass bowl in a larger bowl lined with ice.
Dip the sweet potatoes into the batter using tongs, drain for 2 to 3 seconds over the bowl, and then add to the hot oil. Adjust the heat to maintain between 375ºF and 400ºF. Fry 6 to 8 pieces at a time, until puffy and very light golden, about 1 to 2 minutes. Remove to a wire rack lined with 3 layers of paper towels set over a half-sheet pan. Sprinkle with salt, if desired. Repeat the same dipping and frying procedure with the green beans and parsley leaves. Put the fried vegetables on a serving platter and serve the as an appetizer while preparing the seafood.
Whisk together the remaining halves of dry and liquid batter ingredients as above and repeat dipping and frying with the shrimp and fish fillets. Sprinkle with salt, if desired. Transfer the fish to a serving platter and serve immediately.