Why is taking a deep breath before diving a mistake?

Read the passage carefully and choose the correct answer.

           The excitement of snorkelling in a clear tropical sea is hard to explain to someone who hasn't done it. If you expect to have the opportunity of trying it when on holiday, it is well worth bringing a snorkel if you can get hold of one. Experts now recommend a separate breathing tube.

            Many swimmers take a deep breath before diving with a snorkel. This is a mistake, because it makes the body so light that you dive with difficulty. It is best to take a couple of deep breaths and then a normal relaxed breath in order to breathe in enough for dive. Don't take repeated deep breaths before diving because this can lead to shortage of oxygen, which causes unconsciousness without warning.

            The backs of many snorkellers get badly burnt, for the snorkellers are so involved in the world below they forget about the dangers of sunburn. The sea does not protect your back from the rays of the sun, and it is best to wear a T-shirt. Snorkellers should also consider wearing shoes (or slippers), which may protect their feet from cuts from sharp rocks which can easily become infected.

What does the writer find difficult about snorkelling?

The general impression of snorkelling the writer gives is that________.