John catches Ianto easily, one arm under him to steady him and tilt him up so he can stand again -- the other moving so Ianto can grasp it again.

"Hey, hey, take it easy -- I'm right here. You're good."

So, cool down the flailing for a moment.

"That was good! You were there for a minute, right? You did it!"

Better to draw attention to the momentary success than to the frantic splashing, give Ianto something before he begins his negative spiral.