...but, nothing seemed to happen.
Jon seemed to sense he could still be a mermaid. He just wasn't one right now.
And, since he wished it, the possibility he could be a mermaid seemed like it would be permanent for him, so this was a useful thing to find out with a wish.
Even better, if he could still be a mermaid after the wish, he seemed to sense there were all sorts of other things he could still be if he wished for them.
So, he wished again, and, this time he said, "I wish...."