Jeff looked at Mary with surprise. He hadn't expected that. Then he made a decision and slipped away from her, climbing out of the pool via the stairs at the shallow end, his shark tail trailing behind him as he headed back into the house without a word.
Mary sulked, thinking she had said too much. She floated alone, caressing her skin, marveling in its rough yet pleasant texture.
Then she saw Jeff return, walking with what seemed to be increasing difficulty until he fell to all fours just short of the pool. Mary swam toward that end. "Are you okay?"
Jeff shuddered and his hands clawed at the concrete. Then she saw the dorsal fin emerge from his back and felt a tingle. He'd taken more of the shark vial! A small fin emerged from the back of both of Jeff's forearms. The gray shark skin had spread down to the tips of his fingers and with it came a webbing between those fingers and his nails became more like small claws. Jeff crawled the remaining distance into the pool, his legs weakening as his tail strengthened.
"For you" said Jeff. "To help you fulfill your fantasy."
Mary kissed him, felt his teeth had become sharper. It only turned her on further. Her hands felt lower and found all traces of his legs were gone now and only tail remained - Jeff was now a shark merman . Fins jutted out from where his hips and been and in between them... She caressed his swollen penis. He still had just one, not a pair of claspers, but it felt longer and wider. Mary wrapped her legs around Jeff, and he slid into her.