I am likely among the more liberal Coffee Room denizens and I agree with lilmod ulelameid that this is sick. It is unquestioned even in secular society that it can be uncomfortable for women to wear bathing attire. Here, there is a clear, widely observed religious stricture that coincides with the widely recognized secular sensibility. The state should make accommodations for its religious citizens. In this particular case, Switzerland allowed the girls to wear “tznius” bathing suits, which makes it far less egregious. But it is still a bad idea, IMO, to favor something as random as swimming lessons over religious observances.
As far as the comparison to sex education, I don’t love mandatory sex education either. But at least the case can be made that it is necessary to ensure that children learn the things they need to know and that we know from experience may be difficult to teach. That does not apply to swimmming.