I had a similar issue at my last facility. With around 700 day campers coming into the pools daily it seemed we were having issues every day. Tracking down the culprits, when possible, typically led us to school aged children and not those who would require swim diapers. The kids are having a good time, excited, and often don't realize they need to go until too late. Eventually, I instituted Adult Swim periods 10 minutes on every hour. This served as a kind of a forced bathroom break. I cannot say that it eliminated our issues but it did greatly reduce them.
I'd like to think no one would be doing it intentionally but if that is the case I'm really not sure what you could do to track them down with such a high bather load.
There was another agency in the Bay Area a few years ago that had this same problem, although not 11 times. They were able to trace it back to some lifeguards who apparently didn't want to work and were deliberately releasing fecal matter into the pool so that it would have to close.
We have a 15 minute rest break every hour, so I would recommend some sort of adult only time to force kids out of the pool and to the bathroom.
A while back we were having the same problem, so I started a "healthy water campaign." Now, my facility doesn't see anywhere as many as yours does, but it worked for us. Below is the link, and the article I wrote is on page 18.
Unfortunately, poop happens. I have been fortunate to only have this situation occur 2x this summer so far, but in the past it has been a common occurance. We had one child that seemed to do it every week. You are going about it the right way with informing patrons on your policy, but you may look to update it. I know our county health department now requires the use of a vinyl swim pant for children who are not potty trained. This vinyl pant goes over the regular swim diaper and keeps the excrement from getting out. You can also purchase these in bulk and sell them for an additional revenue source.