The point is that HASHEM is everywhere and that people should not be hypocrites and they should continue doing the same elsewhere as they do at home. In addition they should maintain their level of tznius wherever they go.
It is true that somehow some women forget themselves at the pool because there is a mechitza. They do lie out to dry off and sun tan, and may not realize (not necessarily on purpose) that they are not appropriate to say the least and if their daughters were to do the same they would be very shocked. In addition they should realize that if they do not walk around at home in front of their small children in underwear, the bathing suit does not cover more than that, so as soon as they get out of the pool they should cover up what they keep covered at home. Again, in a more modern environment they are not as makpid, but if you are in an environment where you are makpid and you wear stockings all the time, the pool area should not be different.
EDITED