This happened to me this summer - but I was the guy walking past. Guy was having a wank while looking at a guy fucking his girl i the sea. He was in a very secluded place and I think he thought nobody could see him. I did and he got a surprise at first. However, as soon as I grabbed hos cock he grabbed mine and we both came together in about a minute - it was so horny!! Wish I could have captured it on my iphone.