John had never entertained any mer fantasies like Emily had, but caught up in the lust of the moment he followed her instructions. The substance from Emily's breasts tasted salty not milky. He had no idea what it was.
He quickly began to understand what Emily meant when she said "become like me". His legs spasmed then stuck together. He began to slide under the water. As Emily slipped free of his grasp, John unintentionally took a great gulp of saltwater. He came up gagging as he felt the changes continue. Trying to lift his feet out of the water he instead saw a green tailfin much like Emily's.
"You're becoming a mer like me!" squealed Emily with glee.
She wrapped her arms around John and pushed him onto his back, both of their heads submerged in the swirling saltwater of the whirlpool. Their lips met under the water and briefly John found himself struggling for breath. But then he was breathing easily again, the gill slips having opened on his neck.
John used his newly webbed hands to massage Emily's breasts. She ran her webbed hands along his new tail and sought his manhood, but found it apparently sheathed. Eager to warm John up for another go, this time as mers, Emily slid her way down to John's crotch. She used her tongue to try and stimulate her lover, but was surprised at what she found - and what she didn't. Emily felt John shudder with delight as her tongue probed not a sheathed penis, but a vagina.
Emily broke away and surfaced in the whirlpool. Something had gone wrong! John surfaced as well.
"That was amazing!" said John in a voice unnaturally feminine.
"John, look down" instructed Emily.
John lowered his gaze to see breasts. John's eyes went wide as the realization set in: he'd become a mermaid not a merman!
"The saltwater potion must change you into a mermaid regardless of your gender!"