Mary felt John grow within her. At the same time she could feel her legs begin to weaken as the vial began her transformation. Her legs again turn gray and drew together. John seem confused but enjoying things too much to complain.
Finally both their legs weakened enough from the transformation and they slipped below the surface of the water. Mary could tell by the way that they completely stuck together that her legs had fully transformed into a dolphin tail.
She came to rest on the bottom of the pool on her back, John on top of her. John was completely in the ecstacy of the moment. Mary felt her tail flap gentlely against the bottom of the pool completely involuntarily in rhythm with John's thrusts. They both climaxed and John floated to her side on the bottom.
But they couldn't stay their for long: while they had been underwater longer than they normally could (thanks to the partial transformation, Mary suspected) they still needed air, especially after that exertion.
John "kicked" off from the bottom first, giving Mary a good view of his dolphin tail. His was a little larger and more muscular than hers. His member wasn't visible having slid back inside himself. After a moment to enjoy the view, Mary likewise pushed off the bottom with her tail and surfaced for air.
"That was amazing" she exclaimed.
"I wasn't sure what was going on at first" said John "but then it was too good not to like. Especially with you. So you got some kind of mermaid juice?"
"Not quite: transformation vials. We each had half of the dolphin vial."