I have a question which seems rather silly in the grand scheme of things: Can Muslim men and women swim at the beach?
The basic objective of Islam is the inner purification of a person. Anything which hampers this purification should be avoided. We have been sent in this world to strive to obtain Paradise in the next one. Paradise, as the Qur’ān has specified1, is the abode of pure souls. So if Paradise is our goal, we should always be heedful to our inner purity.
Keeping this principle in view, it seems that Muslim men and women swimming together is something which tarnishes the mirror of their souls. It stimulates base emotions. So Muslim men and women should refrain from it.
1. The Qur’ān says:
He succeeded who purified it and he failed who corrupted it. (91:9-10)