Several things could cause this issue, but my first hunch would be to check the thermostat. However, during a diagnostic procedure, I first check fluid levels, then raw water operation. Check seas strainers, raw water pump impeller and make sure the seacock can open and close freely. If all that is good, then I start checking fresh water components. 160 seems a bit on the cool side to me, and I'm comfortable at 200. 220 is when I start getting concerned and 240 you are doing damage. But the quick and easy, see if your thermostat is stuck open. Good luck and fair winds.