Heat oil in a deep-fryer or large saucepan to 350 degrees F (175 degrees C).
Pat tilapia dry with a paper towel. Cut 2 to 3 slits on either side of the fish. Sprinkle salt on top. Fry in the hot oil until browned, about 2 minutes per side.
Heat olive oil in a skillet over medium-high heat. Saute onion and garlic until onion is translucent, about 5 minutes. Add red bell pepper and ginger; saute until fragrant, about 2 minutes.
Mix water, vinegar, and sugar together in a bowl. Pour into the onion mixture in the skillet. Cover and bring to a boil. Cook until slightly reduced, about 2 minutes. Season with salt and pepper. Add cornstarch. Boil, stirring often, until sauce is thickened, about 5 minutes. Pour sauce over the fish.
