Francis Adirubasamy is a friend of the Patel family. To Pi, he is also known by the affectionate nickname of Mamaji, which means "respected uncle." This gives you some idea of the esteem in which he is held by the young man.
Back in the day, the old man was a champion swimmer, and he's managed to impart his remarkable enthusiasm for this particular leisure activity to young Pi. Mamaji, however, doesn't simply look upon swimming as a leisure activity; it is also a spiritual, quasi-religious practice. For him, it involves establishing a deep, abiding connection between oneself and the natural world.
To Mamaji, swimming involves a love of water. It is this profound, almost mystical love that ensures that, whenever he takes to the water, swimming becomes a meditative practice, a religious ritual that brings one closer to spiritual enlightenment.
Thanks to Mamaji, Pi develops a similar attitude of reverence toward water and the practice of swimming. He finds swimming to be relaxing and peaceful in a way that no other activity is. There is something appropriate, then, in the fact that Pi, or, to give him his full name, Piscine Molitor Patel, is named after Mamaji's favorite swimming pool in Paris.