He works as a diver – how can he fast when water may enter his throat?

Firstly: 

In the
answer to question no. 39232, we have stated that there is nothing wrong with a fasting person
diving into water, but he must take care to avoid letting water reach his
stomach as much as he can. 

Secondly: 

The Ramadaan
fast is one of the pillars of Islam and it is not permissible for a Muslim
to neglect his fast or be heedless about it because of difficulty
encountered as a result of his work. What he must do is strive to combine
fasting and work if possible. If that is not possible and he needs to work,
then he must give priority to fasting, and take a break from work if he can.
If that is not possible and he needs to work, then he should form the
intention to fast from the night before and start fasting in the morning,
then when it becomes too difficult for him, it is permissible for him to
break his fast because of necessity, but he has to make up that day. If it
does not become too difficult and he is able to complete the fast, then he
must do so. 

See the
answer to question no.  43772. 

See also
Fataawa al-Lajnah al-Daa’imah, 10/234. 

Thirdly: 

If a fasting
person swims in water and water reaches his throat without him intending it
to, then this does not break the fast, because he did not do it
deliberately. This is the view of Imam Ahmad (may Allaah have mercy on
him). 

See:
al-Mughni, 3/358; al-Insaaf, 7/434; al-Sharh al-Mumti’,
6/393 

Fourthly: 

The
questioner says that water reaches his throat but it does not enter his
stomach.

 The
scholars differed concerning the things that invalidate the fast – is what
counts whether the food or drink reaches the throat or the stomach? 

Shaykh Ibn
‘Uthaymeen (may Allaah have mercy on him) said in al-Sharh
al-Mumti’ (6/393): The author (may Allaah have mercy on him) mentioned
six issues in which the ruling depends on whether water reaches the throat
of the fasting person. So the ruling is based on whether water reaches the
throat, not the stomach. The apparent meaning of the words of Shaykh
al-Islam Ibn Taymiyah is that the ruling is based on whether the thing that
breaks the fast reaches the stomach. There is no doubt that this is what is
meant, as there is no report in the Qur’aan or Sunnah which suggests that
the ruling is based on whether it reaches the throat, but the fuqaha’ (may
Allaah have mercy on them) said that if it reaches the throat it may be
assumed that it has reached the stomach, or that the ruling is based on
whether the thing that breaks the fast reaches any inner part of the body
and the throat is an inner part. End quote. 

Conclusion: 

There is
nothing wrong with swimming or diving deep into water when fasting. If any
water reaches the throat or the stomach without you intending it to, then
this does not break the fast. 

And Allaah
knows best.