(Maybe he shouldn't have asked that. But she is at least safer now, and some things he was resolutely not thinking about are getting more insistent now.)
“I am birds all the time. I don't know that much about swimming creatures. I'm sure there are much better books on the subject.”
“If you're in a hurry, why not just try doing it? Then you'll know what you need to learn.”
She frowns at him, sighs, and then starts making herself a mermaid tail out of griffin parts that she thinks will be relatively easy to reconstruct later. When she has something that is appropriately shiny and scaly and fits over her legs well she makes a big globe of glass full of air and then attempts to compress it down.
Some discreetly frantic research later, he teaches her how to make a pressure regulator so she can safely breathe from it, and warns her not to dive deep with regular air.
“You need special materials and a particular machine to filter the air. And you also need to watch how fast you go down or up. It's complicated and I'm really not qualified to teach you how to do it safely.”
“It would be pretty silly to make a swimming pool that was too deep, wouldn't it.”
He dry-suit-snorkels along at the surface to keep an eye on her. Not that there are any too-deep places anywhere this close to the beach, but just on the general principle of who knows what she will want to do next.
She mermaids about quite happily and doesn't drown. She seems to find having human joints under the tail inconvenient, and at one point flops out of the water long enough to grab more material from the griffin shell and add it on so her tail is longer and she can get more propulsion without unsightly knee-bending.
He is not cut out for talking people into things. If she won't take his advice he should just give up and — leave —
This is no longer an immediately serious personal safety problem. This is a “who raised this child like this” problem. This is even less his field than diving. He should ask for help.
He starts composing an explanation of the situation.)
Would you like to come with me to get one, or stay here?”
(He will not be especially surprised if she leaves to wherever it is she came from while he's gone.)
“About thirty miles. Maybe fifty minutes there and back if I go by myself or carry you, more than that if you want to fly yourself.”
“‘Just a radio’ can be the difference between life and death.”