John hasn't moved away, so once Ianto begins to flail he's close enough to move inand steady him quickly.

"Hey!" he prompts, "hey, easy. Get your feet down. It's not deep, remember? I'm right here."

So Ianto can grab onto him if he wants, but John is focusing on helping him get upright so he can stand up in the water.

"Easy does it. Feet down, take a breath. I'm right here."

So nothing to worry about, right? John isn't about to let him drown, after all. Especially not in perfectly still shallow water when he's inches away from him.