Once your vegetables and seafood are prepared, heat vegetable oil to 350F (180 C). You can use a deep fryer, or a heavy pan. If not using a deep fryer, use a deep, heavy pot, filled to at least 3"deep. As oil is heating, prepare your batter:
Whisk together dry ingredients. Add egg, whisk until well combined.
Add a small amount of seltzer water , stirring to combine.
Continue adding seltzer water, stirring gently until just combined - mixture can be a bit lumpy.
For best results, set bowl of batter in another, larger bowl that is filled with ice. The colder the batter, the better the tempura!
Gently dip vegetable and seafood pieces in the batter one at a time, slowly removing from the batter and allowing excess to drip off.
Carefully transfer to heated oil. Fry a few at a time - turning every few minutes - until very lightly golden, about 2-3 minutes.
Transfer fried pieces to platter lined with paper towels.
Salt lightly - if you'd like - and serve hot!