John didn't remember that Mary had left two vials next to the pool. He did remember that Mary had used just one vial for their transformations, pouring half into each of their glasses. John concluded half a vial would cause the person to transform halfway.
Like a typical guy, John didn't read instructions or ask for directions. If he had, he would've learned something important about how the vials work: if you drink half a vial, it would only half transform you. But it would transform you based on the form you were currently in.
If you were fully human like John and Mary had been earlier, consuming half the dolphin vial would turn transform you halfway from a human to a dolphin and thus into a merperson. But if you then drank half of another vial it would transform you halfway from a merperson into your new form, which might not be what you were intending.
Not realizing this, John grabbed a vial intending for a transformation into a half-human half-something else. He would not get what he intended.
What vial had he chosen? Would he use it only on himself, only on Mary, or half for each of them? And what unintend form would be the result of his error?