I am both an exhibitionist and voyeur (most of us are to some extent). While there are some great stories here where people have been able to get this to work, in most cases it is very hard to create a masturbation situation with strangers. Females like to look but only if they are in a 'safe setting' and they are not seen or noticed. Seeing females naked and being seen naked is easier than including masturbation.
My greatest success was when I was in college. Close to campus was a rock quarry, which became the local swimming hole. This became my domain. I had strategic places I could oversee what was happening. There were occasional skinny dipping, but more interesting was when individuals or couples would slip away into the underbrush and woods. I learned to follow and observe quietly. It was the first place I ever saw a couple have intercourse (which was much different than movies). It was great to follow the girls, most would slip away to change clothes or to pee. I would follow and watch them and then close to the trail I would do the same thing (appear to be peeing or changing clothes) for them to see me when they returned. They could hardly be upset as they had just done the same thing in the woods as well. By the way, girls pee in the woods three ways. Take off their pants and lean backwards and go; Take off their pants and sit on the edge of something and pee as if on a toilet; or squat, pull their shorts/swimsuit to one side and pee.
My best masturbatory experience was lying on a large flat rock next to one of the trails. I was just nude sun bathing when I heard voices and from my vantage point saw three coeds heading down the trail toward the quarry. They had to walk right by me. I started stroking myself. They became very quiet as I am sure they saw me. I sped up my strokes and vocalized as I cum on my stomach. I lied there with my eyes (mostly) closed pretending not to notice them and they quietly walked by as the sun baked the semen on my stomach. About 20 minutes later, I pulled my swim trunks on and headed to quarry. I went into the water and washed the dried cum off as the 3 girls were lying on a blanket watching. They whispered and giggled among themselves.
I would love to create other situations where I don't look like a perverted flasher, scaring women and yet be able to be watched as I jack off. I have had few other situations but they are very hard to create.