While filming the underwater scene, Alfonso Cuarón held his breath along with Sandra Bullock to make sure he wasn't asking too much of her. He soon found he couldn't match her lung power.
The tank was used to safely simulate Lake Powell. The sides of the tank were coated in green screens so they could animate the lake and the frog around the tank. Fake plants were also placed in the tank for realism and ease of animation. There was a scuba diver on set at all times to make sure the actress wouldn't drown.
The light box or Soyuz capsule were where most of Gravity were shot. However, both were relatively safe. The Water Tank was signifigantly more hazardous and difficult to film in.