Of course, there is a substantial difference between having sex in public where people are unlikely to notice it (say in the water or hugged in a natural position) and openly fully fledged sex among like minded people.
And of course, the question of what sex is comes back. Is holding your erect partner in your hand and possibly caressing or massaging sunscreen in his skin with no intention to make him come off sex? Or is sex happening only, as is the case with them legal definition in most countries, when there is intromission of some part of one person's body into some part of another person's?
Now if you make the answer to this last question clear I might share with you some of my experience.
In my case it was beaches where no one was in sight and there was limited opportunities for anyone to appear at short notice.
Except for the time at a busy suburban beach (textile, naturally) where I got massaged in the water. Once she got the desired reaction within my brief swimsuit she walked out of the water leaving me with the choice of being stranded or showing the results in obvious detail. I waited in the water before things settled down.