How are underwater scenes in movies filmed?

Question

In some movies, for instance the Jaws series, it's a wonderful feeling to see sharks underwater and have it feel very realistic.

I'm curious to know how they film underwater scenes in movies. Not just in Jaws, but other movies as well.

What are the techniques used to film the underwater scenes in movies?

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Arremer · Accepted Answer · 2013-05-27 14:23:21Z

Mostly with underwater cameras and divers mixed with CG.

This particular scene looks to be filmed completely underwater with shark model.Now they would simply film the water surface and add CG shark later...with every year the CG effects grow better and they just make complete scene with CG.
When you need to film actor in water you shoot the scene in the pool and cover the back or above water depends on camera angle with green screen so that later the CG can make it seem they are deep in the water or on some other location.

Behind the cameras there is full team in diving suits.The scene is shoot in many little pieces depending how long actors can hold the breath before then they dive out or are given the oxygen tank.When the video is cut and other camera angles added it appears as they never left the water.

When actor's head can be hidden you can hire the proffesional "breath" divers to shoot the scene in one constant roll.

Here you can see how they filmed harry potter underwater.

http://www.youtube.com/watch?v=F90YDG1zjcI (another video,first one seems to be unavailable)

+1, this answer made me not to post my answer because your answer is better then what i am going to post.
Worth pointing that underwater vehicles are often models shot in fog in slow motion. No water involved.
Divers do not use an "oxygen tank". At more than around 33 feet deep, pure oxygen is deadly. Most scuba tanks contain compressed air, or maybe nitrox "enriched air" which has an increased content of oxygen. Air is 78% nitrogen and 21% oxygen. The most enriched air in common recreational use is 64% nitrogen, 36% oxygen but depth is limited to 95 feet.

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