I'd say a gallon of water would be unmistakable when you drain the oil. It should come out all muddy and nasty. I worked on a 4cyl van that had a hairline crack in a cylinder. When the engine would heat up water would pour right into the cylinder. The engine would cool and suddenly hold its water just fine. I checked the dipstick and it looked like I dipped it in water. (actually I did) When I drained the oil it looked like a storm drain. That was CRAZY!