[Sailor Steve]

That would of course depend on how clear the water was and how deep the periscope can be raised without the seals leaking.

I don't know the answer to either one, and both may be variable, so the only real objection I would have is - are there any reports of it actually being done. I'm certain that hiding that way was done, but was it by periscope or by hydrophone alone? I don't know.