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That is generally more of a recommendation. The recreational diving limit is generally considered to be 130ft, as that's the depth you can't spend more than a few minutes at without incurring mandatory decompression (breathing air).

You can take a deep diving course if you like, but not everyone bothers. Going below 130 though becomes a technical and very complex endeavour quickly.

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This is almost correct.

People imagine that diving training agencies has police power to allow or forbid you to do something. That's not true.

When you get a certification, the agency certifies that you have been trained to go to 18m. That's all it does.

After the training you can do whatever you want. But if you're not stupid and don't want a quick death or a painful life (wheelchaired due to bends), you will stay within the limits of your training.

So it's not only a recommendation, but at the end it works like one.

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