Having spent many days both in colder and similar air temps with an almost identical water temp up here in the PNW, I can tell you that if your counselor is as trained as I had to be they will know their limits, I knew mine was 45 min swimming in only my underwear before I was at the point of needing help (learned under very supervised conditions with a proper warm up after). I completely agree that while they may know their limits they are putting themselves, and possibly others, in danger by not taking a simple precaution. Props to you for keeping an eye on them and for following up and continuing to educate them and others on hypothermia. If you see this becoming a usual issue with all, or a pair of constant ones, of the staff maybe its time to go to the person above and ask that there be a more in depth education process on the risks they are taking.
Thanks for keeping safety on your mind!