bathythermograph
 

Apparently at least some of the subs in the game have one.

I was diving in one of the single missions and heard one of the crew whisper "passing below thermal layer, sir!"

I can't find where the actual gauge is located, but its nice to know we have the ability to detect thermal layers to hide from sonar under them, just like they did in actual US submarines (it was in SH1 also).