What statistics are available for openwater swimming and the number of participants?

Hey, Paul.
I'm not sure which statistics you're looking for? Do you mean how many people are participating in openwater swimming nationwide? The United States Lifesaving Association (www.usla.org) has beachgoing statistics & rescue statistics but it's hard to say how many people do regular open water swimming. USA Swimming has a list of participants in open water, sanctioned races on their website, www.usaswimming.org .
Statistics for participants in triathlons or open-water aqua classes would be different. Hope this helps a little.