Heaven Cent by Piers Anthony

a form changer, you could be a threat to me, so that I might be concerned about sleeping in your company. That would be inconvenient. So let's agree that I will tell you the rest of what I know about the Heaven Cent, and you will not try to harm me. You will not become a swordfish and slice me up, for example. You may do anything else you wish, but you will not offer me any violence or other harm.”
    “I can try to breathe water and escape?”
    “Yes. You just may not attack me, or try to hurt me otherwise.”
    The truth was that Dolph really did not want to hurt her, she was a reasonably nice person, once allowances were made for her adult nature. He just wanted to get away from her. It seemed a fair deal. Except—
    “Will you also tell me how folk summon the—”
    “No. Not while you are a child. You know that.”
    Dolph grimaced. It was infuriating the way all adults hung together; not one of them would break the Conspiracy. But he hadn't really expected to succeed. “All right. You tell me about the Heaven Cent, and I won't try to hurt you.”
    “Agreed.” She extended her hand, and after a moment he realized that he was supposed to shake it to seal the deal. He did so. Her fingers were webbed, but her hand was marvelously soft, and she squeezed his fingers in a way that sent a small tingle through him. More magic, of course, but not really objectionable.
    She moved to the fireplace. “Now I will fix you something to eat. You're a growing boy; you need good food.”
    Dolph didn't like the sound of that. It seemed to be another adult conspiracy: to make all children eat yucky stuff. “What—?"
    “Seaweed soup. Plenty of vitamins and minerals and proteins. As sure as water douses fire, a growing boy needs them.”
    His worst fear had just been realized! Why hadn't he thought to make that part of the deal: no yucky food! Now he was stuck with it.
    As they ate the soup, she explained about the forging of the Heaven Cent. “It can not be Grafted by ordinary means; it requires a very special magic called electricity. This is used to plate the copper on it: electroplating. Only when it is done just so will it function properly. So you will need someone with that kind of magic, and time.”
    “Time?”
    “I understand it is a very slow process. I believe it took two years to make the last one.”
    “Two years!”
    “I agreed to tell you about the cent; I did not say you would like that information.”
    “Well, I agreed not to try to hurt you,” Dolph said, annoyed. “I did not say I wouldn't insult you, fish-rear.”
    She laughed, her merriness returning. “Beautiful, Prince Dolph! We shall get along famously, as sure as sand displaces water.”
    “You aren't mad?” he asked, disgruntled.
    “Of course not. My rear is fishy. But that it merely an option; I, like all my kind, can change.” And abruptly her tail fuzzed, and reformed as a pair of well-fleshed human legs.
    “Oh.” He should have realized. “But then you can go on land. You said—”
    “I said I was a creature of the sea, as indeed I am. Certainly I can go on land when I have to. But I don't like it; the weight on my feet is burdensome, and the dryness intolerable. If I went to the dragon's nest and were lucky enough to get the firewater opal, I would still have to walk back to the sea, and I doubt I could make it. No, these legs are mostly for show and for certain specialized applications; otherwise I prefer the tail.” The legs fuzzed, and the tail reformed.
    Dolph was almost disappointed. He seldom got a good look at bare woman legs; usually all he saw were those of nymphs as they ran away. Nymphs were great teases. It wasn't that legs were special, but that for some reason children weren't supposed to see them too close, and that of course made him curious.
    Then Mela swam out to tend her estate. She had, she explained, a herd of sea cows and a sea horse, which she maintained on sea oats. Also a nice patch of sea cucumbers, which she