I haven't researched the law on this question, but when dealing with the ADA in an employment context, the law requires employers to provide reasonable accommodations to their employees that will allow them to perform their job duties. The key in that analysis is "reasonable." It isn't always the ideal accommodation, or exactly what the employee might most like to have/do. My assumption would be that in a case like this, if the pool is accessible to the individual without waiving the entrance fee for the aide, waiving that fee would likely not be legally required simply because it is preferable.
Or, ditto Dude.
