Does the fast become invalid by the entry of sea spray into the throat but not the stomach, and is it permissible to break the fast due to sudden fatigue and exertion?

There is no harm in a fasting person swimming or diving in water, provided care is taken to prevent water from entering the stomach.

If water reaches the throat or stomach unintentionally, the fasting person's fast is not broken by that, and this is the view of Imam Ahmad.

As for the fast of Ramadan, it is a pillar that should not be neglected merely due to the hardship of work. If it is impossible to combine work and fasting, and the fasting person is compelled to work, then he should form the intention to fast overnight. If he then experiences extreme hardship, it is permissible for him to break his fast and make up for it later. Otherwise, he must complete the fast.