There are many beaches in Greece where nude bathing is accepted, especially on islands which have a lot of tourists (with the exception of Patmos, in the Dodecanese islands).
Sometimes one will find that the beaches in more built-up areas are bathing suit beaches, but that farther down, as the buildings (hotels, rooms, tavernas, etc.) thin out, so do the bathing suits.
A good rule of thumb for areas where one isn't sure what the rules are, is to simply look around (or ask someone who speaks your language). But if there isn't a bare breast or behind in sight, best put on the bathing suit, not just to keep from being approached by police, but simply out of respect for local sensibilities.
As almost everywhere else in the world, not everyone is ready to see people walking around naked, or to have their children see them, natural and holy as the human body may well be. Don't sunbathe half nude (or nude) near chapels and churches, and don't engage in sexual acts on beaches where there are other people (or where others are likely to show up). Not only can such behavior more surely draw the police, but in any case, such acts will be offensive to all but voyeurs and those of pornographic mentality.
There are certainly plenty of postcards, T-shirts, prints, and kitch sculpture seen in touristed areas of Greece (especially on the islands), which present Greece as a haven for sex-sun-sand-sin, etc., and this is something that many would love to see become happily extinct (though it most likely won't).
In any case, it seems unlikely that those who view Greece as an erotic playpen will be reading an article such as this, but instead start heading for the nearest beach where he/she can indulge her/his fantasies. Just in case there's an exception, however, this much could be stated to the open ear:
As a visitor to Greece, you are a representative of the country from which you come, and of the outer world in general in the eyes of the local people. You are not visiting an anonymous playground of the Mediterranean, but the home of a particular people and of a complex, fascinating, and in many ways beautiful, culture.
You are highly visible, and you leave an impression that will become part of the general impression that Greeks have of 'foreigners' and which will affect how they treat you and other foreigners as well. Consider both what you would like to bring to Greece from your country of origin, and also how you would like to be seen (and, by extension, have other foreigners regarded by Greeks).
As to the first, you have the option of bringing sensitivity and kindness to the people of Greece who work unbelievably hard and long hours (many of them seven days a week) during four long hot months in the industry geared towards making your stay in Greece a pleasant and relaxing one, and as to the second-you have the option to be seen as a visitor who not only demands energy, but one who also gives the human gesture of empathy, kindness, sensitivity and respect for local ways.