About the drowning issue, isn’t it a mitzva from the torah to teach your children to swim?? How could it be that so many people-adults (especially in the NY-NJ areas) who never learned how to swim? And it doesn’t end there but the children don’t learn how to swim either!! Parents have an obligation to teach or to pay for swimming classes.
Now about mixed swimming, I have never heard of ANY heter allowing mixed swimming or to go places where people are inapropiatly dressed. Even if the person who isn’t dressed correctly is an 80 year old or weighs 300 lbs it is not allowed. Period.