Anhydrite n screeds should not be designed to falls and should not be used in areas where they will be permanently wet (swimming pools) or in areas where intermittent long term wetting and drying may take place.
a suitable floor covering should be applied I order to manage the risks of wetting in bathrooms etc.
I have heard of people using membranes for wet areas, as manufacturers of Anydrite screeds we don't recommend their use in permanent wet areas or areas of long or persistent wetness. Most wet areas are genarally laid to falls, and this is best achieved with another type of screed