When Bob and Carol place the bombs on the back of Vincent's houseboat they are placed right at the waterline on the outside of the hull. When the bombs go off the entire upper level of the houseboat is blown away, but there is no damage at the waterline at the back of the houseboat. If the bombs had gone off at the waterline at the back of the houseboat, the entire back end of the houseboat would have been destroyed and the houseboat would have sunk. Instead we see a shot of the houseboat still floating with the back end of the houseboat still intact.