The Official Website of Grand Ayatollah Makarem Shirazi

If a fasting person submerges underwater, whether under pure water like drinking water or mixed water like the muddy water of rivers, based on the obligatory precaution, their fasting will be invalidated. The same rule also applies if their body is outside the water and only their head submerges completely underwater. Even when fasting a specific obligatory fast, if one submerges ones ahead underwater for the purpose of ghusl, both their ghusl and their fasting will be invalidated. Therefore, they must both redo the ghusl and observe the Qada for that fast and it is obligatory upon them to pay expiation too. Also, if a fasting person jumps in the water to save the life of a drowning person and submerges completely underwater, their fasting will be invalidated. Of course, saving the life of a person is obligatory even if it causes one's fasting to be invalidated, but one must later observe the Qada of that fast. But if one submerges their head underwater inadvertently, such as when one falls in to water accidentally, or if they forget that they were fasting and submerge their head underwater, their fast will be valid, but the obligatory precaution demands that as soon as they remember, they must get their head out of water. But it is permissible for a fasting person to plunge their body in water. As for underwater divers and frogmen, they can submerge completely underwater while fasting if they wear a headgear that completely encircles their head while they dive underwater. Also, if a fasting person plunges a part of his head underwater while another part of it is still out of the water, their fasting will be valid. Similarly, if they submerge one half of their head first and then submerge the other half, their fasting will not be invalidated. However, it should be noted that the hair does not constitute a part of the head in this ruling. Thus, if one submerges their head completely underwater while their hair is still out of the water, their fasting will be invalidated.