Water in the radiator

Agree. If its not showing up on the ground then its either going out the exhaust or into the engine. If its in the engine it will turn up in the oil pan and turn your oil into something that looks like a milkshake. Check your dipstick. If its going out the exhaust a cooling system pressure test, and/or a compression test should tell you where. You could try adding Bars Leak to the cooling system, but if its loosing that much coolant the leak may go beyond what that product can address. No harm in trying it though, and its a cheap "fix" if it works.