Howbeit in vain do they worship me, teaching for doctrines the commandments of men. Mark 7:7
It's not just mixed bathing that's dangerous, but mixed kayaking, as well. As reported by Ynet News on June 9, 2011:
The summer season is about to begin, along with some unique halachic issues: Rabbi Yuval Cherlow, head of the Petah Tikva hesder yeshiva, has ruled that a man must not work as a lifeguard in a swimming pool where women bathe or in a kayak rental facility where visitors arrive in their swimsuits.
A reader of the yeshiva website, seeking the rabbi's opinion, said he was considering working at a kayak facility in the Jordan River as an instructor who helps visitors get into the water and explains how to use the canoe.
He also wondered if he could work as a lifeguard in a swimming pool where women bathe. He said he had searched the Internet for answers, but could only find questions by women asking if they are allowed to bathe in a pool with a male lifeguard.
Rabbi Cherlow responded that by doing so, he would be crossing a red line. "Working with kayaks means being in a very immodest place all day long," he explained. "This isn't a matter of likelihood but a matter of certainty, and it's just like going to a mixed beach."
The rabbi added that the same applies to a lifeguard in a place where there are women.