I lived in Huntington Beach, CA for awhile back in the early '80s, very near the middle of the beach itself, right near the pier, and an easy walk from our apartment that I walked often that summer I was there.
Whenever I would go, my M.O. was to stroll along the beach and look for some hot girl to spread my towel out beside. (OK, yes, I'm a dog, but hey, why not?)
This was around the time when thong bikinis were rare, but not unknown, and this one day, I spotted a slightly plain but very attractive brunette girl in her early 20's in a black thong lying on her stomach just as I entered the beach. (Jackpot!)
Normally, I would spread my towel kind of alongside bikini girl, but because of the thong, I went downwind, so to speak and spread it out below her and between her and the water, all the better to view her fine, fully displayed, smooth, round, white ass.
Now, this sounds creepy I suppose, but up to this point, I've actually done nothing unusual.
But then, after about an hour, and a couple of good dips in the old Pacific, I got to feeling, how shall we say, frisky?
So, lying on my stomach, I casually reached under my towel and dragged out a handful of sand or two, as if I were trying to get rid of an offending rock or shell. This created a crater in my towel big enough for a little 'undercover' work.
It's been over 30 years now, so my memory is failing me a bit-I want to say that I somehow managed to free my aching hard-on into that depression in the sand, but more likely I just reached into my trunks as subtly as humanly possible.
But whichever, I then masturbated pretty freely, lying on my stomach with the action completely obscured, though obviously not with the gusto I'd usually apply.
At a couple of points on my watch, she got up on her hands and knees, and the view from downbeach was truly something to behold.
At the end of this, um, tanning session, after another fine dip in the ocean to sort things out, I told her that it was about the finest bathing suit I ever didn't see, which got a decent laugh from both her, and a woman she was with her who I assume must have been her mom.