Cut the squid bodies (tubes) crosswise into ½-inch wide rings and leave the tentacles whole. Place them in a glass bowl and cover with buttermilk. Place in the refrigerate to marinate for at least 4 hours, or up to overnight.
Heat your deep fryer to 350°F, or pour vegetable oil to a depth of 1 to 2 inches into a large skillet. Heat to 350°F.
On a platter, shallow bowl, or pan, whisk together the flour, cornmeal, salt, pepper, baking powder, and garlic powder (if using).
Drain the buttermilk from the calamari. Place the calamari back in the bowl. It's okay if some of the buttermilk remains. Dredge the pieces (rings and tentacles) through the flour, shaking off excess, and place them on a large platter or cutting board. Do this in batches and clean your fingers (or fork) from time to time if becoming clumpy.
Working in batches, carefully add the coated calamari to the hot oil and deep-fry until golden brown, about 2 to 3 minutes. Use a basket or slotted spoon to remove them and place them on a platter lined with paper towels. If desired, keep the fried calamari in a low-temp oven to keep them warm while you continue frying.
Serve at once with warmed marinara sauce and lemon wedges.