The once-mermaid knew, of course, that the victim would be in possession of the ring, so he took precautions. He watched Jenna disembark and waited until she was standing near the edge of the dock.
He stepped directly up to Jenna and said, "Hey, take a look at this ring I found!" Jenna, a little surprised as she only barely knew John, nonetheless raised a hand to examine the ring, wondering why a boy would be showing off an ornament like that anyway.
Only then did the mermaid trigger the swap. Before Jenna knew what had happened, the hand that had recently been hers was closing on the ring and pulling it off of his finger.
As a final insult, and to be sure of getting away, the mermaid in Jenna's body pushed the disoriented Jenna into the water. Then she took off. Thanks to the athletic body she had so wisely chosen, she was away before anyone else knew what had happened.
Only now, as he splashed in the water, did Jenna realize what body he inhabited.