"You," John prompts, "and water. I recommend letting yourself float a little. You'll want to tense up at first and that's okay, we'll stay where you can put your feet down and I'll be here."

Ready to help if he starts splashing and flailing around. Ianto isn't likely to drown. Honestly, it's far more likely he'd drown John first by getting into a panicking and pulling him under -- but John isn't going to tell him that.