A 'thermocline' is the term used to describe an abrupt temperature gradient in a body of water such as a lake or sea, marked by a layer above and below which the water is at different temperatures.
Check out the temperature variations today from one side of Fresh East bay to the other. The average was 11.7 degrees until reaching the far, more exposed end of the bay, which gets the full force of the oncoming waves, and thermal currents. These are the currents that boost the overall sea temperature and bring all the nutrients. I noticed this a lot last summer, where blooms of moon jellyfish appeared in very specific areas, which often seemed to be warmer.
Today I noticed the water in these same areas was markedly warmer than the other side of the bay. From 11.7 degrees to 13.4 degrees! Summer's coming!