the Fatwa Department Research Committee - chaired by Sheikh `Abd al-Wahhâb al-Turayrî
The critical matter here is the intention. If he swims in the water without having an intention of removing his ritual impurity, then he will not be considered as someone who has performed wudû’ or ghusl.
The Prophet (peace be upon him) said: "Actions are only according to their intentions, and each person will have only what he intends." [Sahîh al-Bukhârî and Sahîh Muslim]
Therefore, if while swimming a person has the intention of removing ritual impurity, then he will not need to offer a separate wudû’. According to the view that rinsing the mouth and nose are obligatory for wudû', he would have to make sure to do this as well.
This opinion that full immersion suffices for wudû' is held even by those scholars who say that it is obligatory to performing the acts of wudû' in a particular order, like the scholars of the Shâfi'î school of thought. This is because a person who is completely immersed in water will have all of his body parts touched for a duration of time, so this condition will be effectively fulfilled.
It might also be mentioned that the majority of scholars are of the opinion that a full purifying bath (ghusl) takes the place of wudû’. This means that the specific motions of wudû' are not necessary as long as the conditions for the bath are fulfilled. They support their opinion with the following evidence:
1. Allah says: “Approach not the prayers…in a state of major ceremonial impurity except for one who is just passing (through the mosque) until after washing your whole body.” [Sûrah al-Nisâ’: 43]
This apparent meaning of this verse indicates that bathing is sufficient as purification for prayer, since neither wudû’ nor anything else is mentioned as a condition for purification.
2. Some people came to the Prophet (peace be upon him) and asked him about ghusl after sexual intercourse. They told him that they live in a cold place. The Prophet (peace be upon him) told them: “It would be enough for the one of you to pour water over his head three times.” In another narration he said: “For me, I pour water over my head thrice.” [Sâhîh Muslim (328)]
The obvious meaning of this hadîth is that performing ghusl where the whole body is touched by water suffices for wudû'.
And Allah knows best.
Source: Islam Today