In the water tank sequence Cranston dives to the bullet holes to get air. When he dives down there is a camera shot looking up at him from the bottom of the tank showing that there is plenty of air left at the top, no doubt left there so that the actor would not drown. And; for the sealed tank to fill completely there must have been a way for air to escape rapidly from the top. When the tank was full water kept pouring in and would have escaped the same way. When Margo arrives there is no water coming off the top of the tank.