By Patricia Scanlon Experienced scuba diver

Open Water is based on true events

Open Water, a film about a couple who go scuba diving only to be left alone in shark-infested waters, is very disturbing, because it's all within the realm of possibility.

The movie will be scary to non-divers, because they're not familiar with the things that divers encounter.

But it will be particularly frightening for divers because they understand just how realistic it could be.

'Demonised' sharks

In this film, the cameramen and the actors were in the sea with real sharks, and I think that translated wonderfully.

It didn't have that computer-generated effect look to it and was much more scary as a result.

The film works so well because of the fear of being left Patricia Scanlon

It's a very scary movie, but it's not a horror movie. It's more a psychological thriller.

It progressed from seeing the divers when they first surface, joking about being abandoned and gently chiding each other about being lost.

They see other boats and they think their boat is with them. But as reality sets in they go through an entire range of emotions. They're kidding, and then they're uncomfortable, and then they start bickering - it's a tremendous psychological progression.

One of the things that crossed my mind whilst watching the movie was that it'll be bad for scuba diving, because people will be afraid.

But also, once again sharks will be demonised - as they were after Stephen Spielberg's 1975 film Jaws nearly 30 years ago.

You can't imagine how beautiful sharks really are. They glide. It's easy to understand that they have cartilage, not bone. You will never see anything move with such grace.

They are not monsters. Jaws had the whole thing about the shark taking it personally and coming back to get them. That is not it.

No control

It is like if you throw someone in a pit with crocodiles. Crocodiles don't hate humans - they just eat things.

So if you are bobbing around in the ocean, and there are a large number of sharks, the chances are that they're going to develop at the very least a strong interest in you, and at the worst, they will probably attack and bite.

Director Chris Kentis filmed real sharks as the "special effects"

I have spent time in the water with sharks. Actually when you're scuba diving, sharks don't bother you too much.

What I thought was very impressive was that even though there are only two possible endings - they either get rescued or they are killed - the conclusion is still surprising.

I never would have expected it.

But the film works so well because of the fear of being left.

I think that's a very human fear.

You get the idea that the two, if not control freaks, are certainly in positions in their life where they control things.

This is such an incredible departure for them, because rather than being the movers and shakers controlling their lives around them, they are left in a situation where they have just about no control.