Is it permissible for a fasting person to hunt fish by diving into the deep sea, with the possibility of water entering his stomach, and is his fast valid? What if his job is to hunt octopus in this manner?

Fishing is permissible in principle, as Allah Almighty says: {Lawful to you is game from the sea and its food as provision for you}. Fishing by diving is a matter of scholarly disagreement. Some have permitted it on condition that water is guaranteed not to enter the throat or ears, while others have deemed it reprehensible.

As for water unintentionally entering the throat, the fast is valid according to the Hanbalis and some Shafi’is, and no expiation is required of him. The majority of scholars, however, maintain that his fast is invalidated, citing the Hadith of Laqit ibn Sabirah: "And be thorough in sniffing water into the nose, unless you are fasting." Al-Iraqi favored the view of invalidation.

If it is probable that the diver will swallow water, then he is prohibited from diving while fasting so as not to invalidate his fast.