Just hold still. [He has no damn idea, really, but he's not stupid enough to admit to that while he is trying to keep her calm. Flailing can't be helpful on a rescue mission.
He very vaguely recalls something he's seen a long time ago - maybe on Baywatch - and starts to turn around so he's on his back, keeping his sight on her and making sure her head stays above the water as he starts treading water again, this time in a slightly better position to use his speed for something good, although he doesn't dare go too fast, this is still all very new.]
we'll have to rely on the age-old art of describing stuff
He very vaguely recalls something he's seen a long time ago - maybe on Baywatch - and starts to turn around so he's on his back, keeping his sight on her and making sure her head stays above the water as he starts treading water again, this time in a slightly better position to use his speed for something good, although he doesn't dare go too fast, this is still all very new.]