All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
There is no doubt that mixed swimming pools where both men and women swim at the same time are a great evil; a Muslim should avoid it, let alone swim in it while both men and women are present.
It is inconceivable that the pool in the club is so big that one does not see others in it, and does not see women. Even if it were assumed that this was the case (a swimmer does not see another in the pool because it’s too wide), the presence of your husband in such a place when it is attended by women is a door to temptation. Hence, it is better for him to close that door because a Muslim is ordained in Islam to protect himself and his family from temptation and its causes. Allaah says (what means): {O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are [appointed] angels, harsh and severe; they do not disobey Allaah in what He commands them but do what they are commanded.}[Quran 66:6]
For more benefit, please refer to Fatwa 82897.
Indeed, there is nothing equal being on the safe side when it comes to one's religion, and your husband should not endanger it for the sake of swimming there. Indeed, he may practice other sports instead of swimming.