"You're not going to drown. Here, just -- keep hold of my hand and try to float on your back."

He runs an arm around behind Ianto, bracing him ready to take his weight. Maybe this will work? Maybe not, he'll see.

"You will feel like you're going to sink if you're tense, but I'll hold you up -- alright? Trying to breathe, to relax."

Then, eventually, he might calm down enough to float on his own.