Thaw fish and seafood, if frozen. Prepare Tempura Sauce.
For batter, in a medium bowl, combine the egg yolks and water. Add flour, gradually stirring in with a folding motion until just moistened. (Don't overstir; that's the secret to a light batter.) Batter will be lumpy.
Heat oil in a large, heavy saucepan to 375 degree F. Dip fish and vegetables in batter. Gently place in oil, a few at a time. Deep-fry for 3 to 5 minutes or until light brown. Drain on paper towels. (Keep warm on a baking sheet in a 300 degree F oven while you fry the remainder.)
Serve the fried fish and vegetables warm with Tempura Sauce. Makes about 20 appetizer servings.
If you can't find the sauce ingredients, combine equal parts white vinegar and soy sauce for a dipper. Or use a purchased dipping sauce that you'll find in the Asian food section of your supermarket.