Just watch her and touch yourself

You sneak behind a tree trunk and crouch down, a good vantage point to see her from. You unzip your pants, letting your cock out in the cool breeze. It's a little hard to see her properly in the shade, but you see her curves, reminding you of a Greek goddess. Her hair is long and knotted, and a seaweed green, but that must just be a trick of the light. Honestly, there's not much to see, but that's not the point of this little voyeuorism. It's about getting off from someone who's unaware of you, gratified in the fact that you used them without them knowing.

You stroke your rock hard cock. It's almost as if her singing was helping you. You feel warm and safe with each touch of your flesh, throbbing for release already. You pant like a dog, stroking more madly and wildly, picking up pace, stroking and stroking, your hand twisting as it glides down to your shaft, her voice like another pair of hands helping you... A wayward twitch sets you off, cumming streaks that dribble over the earth, your hand still stroking to get every little drop out of you. You moan louder than you expected, catching the woman's attention as she stares at you with two large yellow eyes. You're still shaking, trying to stay still behind the tree and orgasming, hoping she didn't see you cum to her sight, but you have no doubt she knows you're there.

She smacks her plump ass, and blows you a kiss along with a wink, before she drags herself on the ground and into the water, disappearing into the rivers murky depths. You swear her legs were webbed between the thighs and calfs, as well as her arms that linked to the side of her chest. Was she... A mermaid? Perhaps the infection affected more than just humans.

Before long the water goes quiet, leaving you once more in the silent forest.