What should woman wear in front of other women in swimming pool?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Visiting swimming pools is a permissible matter if it is in the limits of the Sharee’ah. Some of the limits of the Sharee’ah that should be considered are as follows:

1. A woman should cover her ‘Awrah: from the navel to the knee in front of Muslim women, and her whole body in front of non-Muslim women save her face and hands till wrists and what could be unveiled at work.

2. There should be no mixing with opposite sex.

3. The other women at that place, more likely, should be covering their ‘Awrah.

4. The place should be safe in such a manner that men cannot see what is inside.

5. Permission of a husband is a must for a married woman since his obedience is an obligation and going to a swimming pool is only a permitted act. So, the obligation should be given priority.

As for the Hadeeth in which the Prophet said: “If a woman puts off her clothes in a place other than her husband's house, she tears the veil between her and Allah.” [At-Tirmithi, who graded it as Hasan, Abu Dawood, Ahmad and ad-Daaratqni]; then this Hadeeth means any place where a woman is not sure of being covered from men.
Therefore, many scholars have permitted women to enter the Sauna peculiar to women if they put the above conditions into effect. Unfortunately, many of them do not accomplish the above conditions.

Allah knows best.