Underwater Sequences
Above or below the surface water is an uncooperative medium. Boats float off-station, lines and gear become entangled and props need to be heavily weighted. Nothing is easy to manoeuvre under water. Allow days, not hours, when estimating schedules.
Contamination
As many coastal seas in the Western hemisphere are too murky to permit underwater filming, sub-aqua sequences are invariably recorded in studio tanks or swimming pools. In studio tanks the water must be treated with chemicals to prevent the formation of green algae which grows rapidly in warm water under artificial lighting.
Foreshortening
Water has different optical properties from air, and the pool or tank which would seem adequately proportioned on the surface will appear narrowed and foreshortened below. The rear wall comes forward, the floor tilts up and the sides converge.
Natural growth
Tiled or concrete walls can be disguised by covering them with artificial growth. This is best done by attaching real weed, torn plastic, shredded sacking or painted cloth to garden netting.
Pre-washing
Items of scenery which have been prepared above water will carry a certain amount of dirt and dust. Everything to be submersed should first be thoroughly hosed down on clean concrete or a tarpaulin. Failure to do this could postpone filming for many hours while the water clears.
The sea bed
Sacking or hessian painted and treated with blobs of plaster can be used to cover the bottom of a tank. Loose material such as sand should never be used where it would clog the filtration plant.
False floors of this kind can be weighted down with clean rocks or stones.
1. A glass-sided bubble tank
Bubble-tanks are placed in front of screens to Imply that the action is taking place underwater. Air pressure applied to fish-tank aerator blocks will produce miniature bubbles.
2. A dry submarine
Shot against a backing which is light at the top and dark at the bottom a model submarine will appear to be underwater. Rippling light on the under-side is created by shining a lamp onto pieces of broken mirror placed in a shallow tray of water. To obtain the effect the surface of the water must be gently agitated.