Well, check out the Wiki article on bathythermographs:
http://en.wikipedia.org/wiki/Bathythermograph
Here's a chart from the USS Drum's 9th war patrol:
http://www.drum228.org/warpatrol09.html#density
At a certain depth a large temperature change (thermocline) could be noticed if you watched the bathythermograph.
The photo I posted in my previous post is the bathythermograph in the USS Drum, and it's still showing the outside temperature today.