Question: What is the ruling of swimming in the sea or lakes during the daylight hours of Ramadhaan?
Answer: There is no harm for the one who is fasting to swim in the sea or lakes whether the lake is deep or not. However he should be mindful in not allowing the water to creep into his body cavities to the best of his ability. This action of swimming revitalizes the one fasting and obligates the person’s fast to do what is pleasant so long as it is in accordance to the obedience of Allaah. So it is not objectionable because it is something which makes the act of worship light on the one worshipping as well as making it easy for him.
Allaah, the Most Blessed, Most High, said concerning the verses of fasting:
“Allaah wants for you ease not to make things difficult so you may complete the number of days and glorify Allaah for having guided you” - Al-Baqarah (2):185
And the Prophet (صلى الله عليه وسلم) said:
“This deen is easy not meaning to be hard on anyone except for the one who makes it hard on himself.”
So there is no harm in swimming in the lake like there is no harm in bathing oneself in the shower, or anything similar and Allaah knows best.