This is a great question, Kelly. I know that more and more facilities are facing this issue. Look forward to seeing what others have to say. Here's what Fairfax County Park Authority, one of our Best of Aquatics winners, does.
My first question is what is the justification for your rule of swim suits only? Do you have the rule because you've always had the rule or it is a specific health code? Do you allow people to wear rash guard type shirts? What about a wet suit? I think you definitely need some guidelines for what is appropriate and what is not (transparent clothes, clean edges, etc.) and showering before entering the pool is a must (including the clothes that are going in the pool). The reason I say all this is to look at why you are doing what you are doing with your rules and what are you trying to accomplish.
I don't know of a specific law that supersedes your park rules except for the possibility of something like a discrimination claim. For instance do you have a rule that says no Jewelry but don't strictly enforce it for things like small necklaces (which may be from other religions)? I'm not trying to accuse you by any means of discrimination but asking from a perspective of looking at the bigger picture. You have a rule that clashes with a specific religion and you enforce it. Do you have other rules that are "minor" and don't enforce as strictly? If you are very consistent in all your rules you should be ok, but I think you need to have a really good justification for your rule (or the local code needs justification).
I know one pool in Portland has a males only swim and then a females only swim to help with this issue. They staff only the appropriate gender during those swims. Here is a link to their program. This is one of their smaller community pools.
BTW-I'm not a lawyer I'm an aquatic professional. Please don't consider this legal advice and consult your facility Legal Council.
I run the aquatics department at a Jewish Community Centre, and our orthodox members also prefer to swim in more conservative attire. We have accommodated this need with, as another poster mentioned, a girls and boys only swim hour once a week. These swims are very popular!! We also use our discretion when it comes to swimming attire asking that it is clean, not bulky with wraps or straps etc. that could become an entrapment issue, and it is not restrictive to prevent the wearer from swimming.
There are special retailers who design and sell suits to these populations.. if I can find a link I will post it.
Kelly - I think I've shared this info on AI before.... most of us have come across this situation to one degree or another. The pool facility I manage is within a school, there had been a PE requirement that prompted a discussion between myself and the father of one female student about how to respect the applicable religious and cultural requirements for modesty, in our community program we also have a number of families who needed the same considerations and I have one girl on my lifeguard staff who falls into the same category. What I have come to understand is that there is a wide variance in individual compliance with religious rules as with most religions, and some families are more relaxed than others, but common sense and communication solve a lot. The wind up for us is that we require "proper bathing attire" as per the NY State Sanitary code and define it as clothing made from a material that is meant to be in water. Wet suits, rash guards, board shorts, womens bathing suits, even some under armour shirts all comply without running dye into the pool, soaking up too much water creating a slip hazard on decks or in locker rooms or becoming billowy in the water creating a potential entrapment hazard (tangled swimmer or caught in a pool drain). A combination of these types of articles and bathing caps can cover as much or as little as the individual requires. The posted rule has always specified "NOT cut off shorts, tee shirts or other clothing". Usually enforcement is a situation of initial notification by the lifeguards and often a calm personal follow up from one of the senior guards or myself. But, anything with strings or excessive loose fabric on ANY person is unsafe due to possible entrapment, that swimmer must change or not be allowed in.