"I have an idea" says Amy. She takes you by the hand and leads you to the school's indoor pool. You want to dive right in, but you and her both know there's too much of a chance of getting caught. Instead, she takes you into the girls' locker room. It too is empty, but less likely for anyone to find you there. Just in case, the two of you duck into one of the showers and pull the curtain closed.
Finally with some privacy you resume making out. Amy reaches up under your shirt to caress you breasts. They're straining against the fabric already so you take off your shirt, then reach under her shirt to remove her bra and return the favor. You kiss and grope and grind, but something seems off. You see the shower knob and realize what you're missing.
You turn on the water and step back so that it sprays you down. It feels so good as the red scales began to appear on your legs. You almost forget to remove your shorts and boxers before they're torn by your transformation. You barely get them off and cast them aside before you sink to the floor of the shower, your legs merging into your mermaid tail. You sit there, head thrown back, water cascading down your chest and across your tail.
Looking at Amy, you see she has stripped off her clothes and has been masturbating as she watched you change. You drag yourself over to her and move her hand aside, licking her fingers. Then as she leans against the wall you slide your tongue into her. She moans, both from your pleasuring and her own mermaid transformation as your wet torso presses against her legs. Soon you have to break off what you're doing as she can no longer stand. Amy tries to slide down the wall, but ends up partly falling slowly onto you. The two of you are left laying on the floor of the shower, tails entwined, water still raining down on both of you.
"When I accidentally transformed this morning" you say to her "I pleasured myself to the thought of us being together again like this.