There is nothing wrong with a fasting person diving into water or swimming in it, because that is not among the things that break the fast. The basic principle is that things are permissible unless there is evidence to show that they are makrooh or haraam. There is no evidence that swimming is makrooh or haraam, rather some of the scholars regarded it as makrooh lest anything enter the swimmer’s throat without him realizing it. End quote.
Fataawa Ibn ‘Uthaymeen, 19/285
He also said: There is nothing wrong with a fasting person swimming, and he may swim as he wishes, and dive into the water, but he must take care to avoid letting water get into his stomach as much as he can. End quote.
Fataawa Ibn ‘Uthaymeen, 19/284
It says in Fataawa al-Lajnah al-Daa’imah, 10/282
Swimming is permissible during the day in Ramadaan, but the swimmer must take care to avoid letting water reach his stomach. End quote.