At a public park with a small, pleasant lake fed by a mountain stream, my wife and I were on one side in the grass,we lay on our towels as normally done on beaches or lakes. My wife is young and beautiful with a great figure and wears tiny bikinis. Not hard to miss, across the lake is a young man with a camera or two with telescopic lenes aimed across the lake. Not being a rocket scientist, I took about two seconds to know he would probably find my wife in his viewfinder and take pictures like a paperazzi. Neither of us really care and she even decided to give him a good view a few times letting a breast fall out here and adjusting the bikini for a Sharon Stone style photo. We actually chatted with the photographer after swimming over to that side and just asked what he did. He said he like to take pictures of people and they were artistic for his own use. We did not confront him at all since we are adults and felt no harm was done, even with the accidental pictures taken without permission and with no release. My wife is not a celeb or in public trust positions like a teacher.  Now, I have a big question.

I was told that on public property people can take pictures of those using the public property, without getting their permission. I to know if this is true.
It seems to me this is OK. My wife was a little too generous maybe, but she made the decision not the photographer.  However, as is well know now some people like to take hidden photographs out on the street looking up the skirts of women. From what I read, this is illegal and can get them arrested.  My local Y has signs all over the locker room doors to warn people to NOT use cameras and phone cameras in the locker room. Also, there are signs warning that children over three (3) must only use gender specific locker rooms or go with family to the family dressing rooms.  That is understandable as this is preventing taken pictures of children's bodies without anyone's permission.  OK, now when the photographer goes outside to a pool, what happens? Suppose he starts taking pictures with his cell phone camera,of teen age girls who might be 18, but more than likely are HS kids looking like adults, as they often do. He is not asking permission, is doing it differently than the guy with a big old telephoto lens (if that matters) because his little cell phone is hard to spot out in the pool and he is just taking pictures of what everyone can see. Nubile, young girls in bikinis who presumably wear them for the attention given to their bodies in bikinis.   Is THIS legal?  Could a cop be called and arrest the man for taking pictures of the girls even if he has no idea they are under 18.  I have to admit that the age 18 thing is only seen on sites like this one restricting content from children

Well, this is a long one, but I hope it says eveything to make some sense of laws and rights hat seem to protect privacy and those that protect the freedom of individuals to do lawful things.  I hope we get and answer   Thank you.