Slice the 2 Vidalia onions or 1015 sweet onions into 1/2-inch rings. Set aside any very small rings for another use.
Place 2 cups flour in a shallow bowl for dredging.
In a large bowl, combine 2 cups buttermilk and 1 cup water; whisk to blend.
In a separate bowl, mix 2 cups fine cracker crumbs and 2 cups white cornmeal.
Pour vegetable oil for frying into a deep pot or deep-fryer to a depth of about 2 inches (enough to submerge rings). Heat oil to about 350°F (175°C).
Working one ring at a time: dredge the ring in the flour, shaking off excess; dip it into the buttermilk-water mixture, allowing excess to drip; then press both sides into the cracker crumbs–cornmeal mixture so the ring is evenly coated. Place the coated ring on a tray or plate.
When the oil reaches temperature, fry the rings in a single layer in batches (do not overcrowd). Fry each batch about 2–3 minutes, until golden brown, turning once if needed.
Use a slotted spoon or spider to remove rings and drain them on a wire rack set over a baking sheet (or on paper towels) in a single layer.
Repeat steps 6–8 until all sliced rings are coated and fried.
Let drain briefly on the rack, then serve warm.