In theory that's correct. However it would be quite wrong to assume that a sub would
always be detected. Many, many factors would influence detection, not least water temperatures.
Coincidentally, was just looking at:
http://www.uboat.net/articles/index.html?article=45
Interesting Eh.? This could be one of the reasons why a prominent U-Boat Commander ( I
think it was Erich Topp ) wrote in his memoirs that he seldom, if ever, submerged below 30 meters. If so , I wonder what he did in the Winter?? Maybe he hibernated

!
Anyhow, as Privateer says, discussion of this is pretty academic. This sort of stuff isn't modelled in SH3. More's the pity!