They work, they lower the evaporation in still water, i.e. when the pump is off.
When the pump is on, the monolayer is disturbed and when swimmers are in the pool that gunk of fatty acids (bacterial food, biofilm layer in waiting) it will coat the entire filter providing the ideal environment for growth of pathogens, usually the thing we are tying to avoid. In Europe the owners of the product change frequently and I have heard of court cases after the water went bad but the settlements have a gagging order attached so you don't find out much about them. We have enough problems with sun cream without adding it on purpose.