Once John's changes finished Mary brushed up against him and they nuzzled together. He clicked and squeaked at her.
'Dolphins?'
'Always loved dolphins. Smart. Beautiful. Have sex for fun like people.'
They swam around the pool - playing, rubbing, nipping. Finally Mary rolled onto her side and floated still. John swam up, first rubbing his snout on her slit then moving up and sliding into her. It was brief intercourse, but the sensations were different. For Mary it was still a dream come true.
They had just finished and were nuzzling when the pool's cleaning system started up.
'Time go back' squeaked Mary.
'Was wondering' replied John.
Mary felt her flippers lengthen into arms. Her view shifted as her neck reshaped. She felt her legs growing back...but still felt her tail in place. What was going on? Turning her head she could now look down and see her legs and arms which were still dolphin gray. Mary swam to the pool's stairs and struggled out of the water and examined her body. Her feet were like flipper slippers without distinct toes. Her hands had defined fingers but with a membrane of skin between them. Mary watched John struggle out of the pool with a similar from. Mary explored with her hands, finding her blowhole gone, but her dorsal fin and tail still in place.
As Mary intended, the cleaning system spread the human vial through the pool's water. However, she hadn't really thought things through. The vials were meant to be ingested completely for a complete transformation. The human vial had been diluted so its effect was far from complete. Mary and John had returned to only slightly human form; they were mostly still dolphin.