He rings up scuba equipment; the level of detail is unnecessary but it's the first thing that comes to mind, you can drown in a puddle if you land wrong, and he breathes, feeling his limbs relax as the water stops being affected by - was that magic? - and only reacts to his flailing movements.

He waves to Septimus, and then, eyeing the knife, gets up by himself, water dripping off him. Muffled, he says, 'Hi! Don't mind me, I'm busy drowning.'

Because the right way to deal with an opponent more powerful than he might be himself is clearly to be a little shit.