In answer, he takes a slow step deeper into the water.

"Can you swim at all?" he prompts, because it's possible Ianto can't -- but it's also possible he can and just had a bad experience, or is nervous in anything that isn't a pristine man-made pool. "It's fine either way, we can work with it."

There's no judgement in his tone, either. He doesn't mind. It's something that's important to learn, so he can be patient.