I have heard that, when you are swimming in the pool and some water enters into your body through your nose and you can feel that in your mouth.
actually i felt it while swimming.
So, the question is:
Does it affects the fast?
First what breaks the fast is eating, drinking or swallowing anything that can be considered as food or beverage or having sexual intercourse during the daytime of Ramadan or the day you fast (note that some schools of jurisprudence say that anything -which is considered to be food or beverage- that enters your body be it through the mouth, nose or any other entrance as breaking the fast). So swimming doesn't break the fast by default, but while swimming you risk to swallow water and therefore it is not recommended to swim while fasting.
But there are basically two possible verdicts depending on your abilities: scholars say if you are a good swimmer and you can guarantee not to swallow water (that will reach your stomach) you may swim while fasting:
Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said:
There is nothing wrong with the fasting person swimming, and he may swim as he wants,and immerse himself in the water, but he must be careful to avoid letting water enter his stomach, as much as he can. This swimming will energize the fasting person and help him to fast, and whatever gives a person energy to carry out Allaah’s commands should not be disallowed, for it is somethingthat makes it easy for people to carry out His commands. … (source: islamqa #65734)
While if you know that you usually swallow water while swimming it is not allowed to swim for you:
Al-Adhra’i(one of the Shaafa’i fuqaha’) said:
If he knowsthat water usually enters his stomach when he immerses himself in water, and he cannot avoid it, it is haraam for him to immerse himself. End quote.
Haashiyat al-Bujayrami (2/74). (Source: same fatwa from above)