John wasn't about to simply give up hope, and began frantically "kicking" with her tail, trying to chase down the boat. Much to her surprise, she soon got the hang of it, and things got better when she realized that she was designed for an undulating swimming motion rather than the kicking she was used to as a human. Once she switched to that and ducked completely under the surface, she was fast.
But not fast enough, unfortunately. She was streaking along faster than she could ever swim as a human, but as she watched the boat's keel and prop from under the surface, she could see that she was still losing them, just not as rapidly. Regardless, she pressed on, only coming to a stop when she simply could not swim any farther. Completely winded, she watched helplessly as the boat receded into the distance, drawing deep lungfuls of water and marvelling that she wasn't drowning.
There was a lot to marvel about, in fact. The fact that she could breathe water as easily as air was chief among them, but other things stood out as well. Her body, for instance, was neutrally buoyant; she didn't float to the surface like a normal person, but she didn't sink, either. She merely hung there in the water, waiting for the energy to move herself up or down. Then there was her tail, which, unlike the legs she had had as a human, proved to flexible along its whole length past her hips, rather than at a few joints; with practice, she found she could contort it into just about any position imaginable, though she wasn't exactly eager to experiment.
Of course, to John's mind, though it was probably the least drastic change from a biological standpoint, the truly earth-shattering difference was that she was female. Everything about her new body was feminine, from the long, flowing blond hair that would've hung down to her waist if it wasn't floating in a disarrayed mass around her soft, feminine face to her slender, curvy figure (even her tail tapered in a graceful curve from its base to its tip) to her perky, uncovered, sizable-but-not-gigantic breasts. Working up her nerve, John carefully pried open the scaly slit between her hips (or what was left of them; they now served primarily as anchor points for her tail muscles, though her widened-for-childbirth female pelvis still gave a cute, rounded look to what used to be her buttocks,) discovering that she possessed the reproductive organs of a human woman, despite her fishy lower half. She wasn't crazy about having periods, particularily not underwater, but at least she wouldn't be laying eggs.
And anyway, John didn't plan on staying this way long enough to have periods, not if she could help it. The boat was a rental she and several of her friends had taken out to sea for an afternoon of sun and fun, and though the mermaid who now inhabited her old body had driven out of sight, John knew where it would have to be returned. She would head there; if the boat didn't head directly back, as she guessed it wouldn't, then she could beat them back to the marina and alert her friends to the situation when they got there. Of course, with the former mermaid having the body-swap ring, who knew what would happen, but anything was better than just giving up and staying a mermaid. Yes, that was what she would do.
After she got something to eat, anyway. John had burned a lot of energy in that futile chase, and she was quite hungry by now. She hated sushi, but that was what was on the menu, so, after a good stretch to limber up her muscles, she swam off in search of some fish.