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.