You might try first turning the system off and on. Secondly, you might try resetting the calibration point down a few degrees and then see what happens.

In our prior B&G system (about twenty years ago) and our current Hydra system, the seawater temperature funcition is attached to the speed thru the water impeller. I assume that the you have the impeller inserted. If so, you might pull it (and replace the plug ;^) )))) --- then see if you get some sensor information by inserting the plug in some "colder" water in a small cup. See if the sensor responds.

Then, I think its time to call Florida for B&G's service group.