Hygiene....I agree with salla.
If they have a bikini on or normal swimsuit and are having their period, then almost certainly they will be wearing a tampon, which is hygienic in the context of this thread.
If they are completely covered or has a burkini on and having a period, then she MIGHT be wearing sanitary towels and in my book, in a swimming pool, that is unhygienic.........particularly if there are sharks in the pool that can smell blood from miles away.
There is nowhere in the Koran which says that the female body should be completely covered.
Muslim men do not swim in their thobe's, they swim in bathing trunks.
It is female exhibitionism again.
They either adhere to the customs in the society in which they live or "stay indoors" as they would be required to do had the still been in their native Muslim country.