In medium colander fitted over medium bowl, toss together carrot, zucchini, onion and salt. Let stand for 10 minutes, squeeze out excess moisture and discard liquid. Return carrot mixture to bowl. Stir in rice.
In large bowl, whisk together flour and baking powder. Whisk in egg until blended. Gradually whisk in soda water (do not overmix; it’s OK if batter is a little lumpy). Stir carrot mixture into batter.
Meanwhile, in large saucepan or high-sided skillet set over medium heat, add enough oil to reach 3 inches up sides of saucepan. Heat until shimmering or until instant-read thermometer registers 375°F.
In batches to avoid crowding, scoop 1/4-cup portions of rice mixture and carefully spoon into hot oil. Cook for 2 to 3 minutes or until fritters are golden brown. Transfer to paper towel–lined plate to drain.
Serve fritters with tzatziki sauce for dipping.
For variation, these versatile fritters can also be served with warm marinara sauce, guacamole, sour cream, garlic mayo or chipotle mayo for dipping.