Question for anyone who help can answer. I traveled across Lake Michigan last weekend at 3600 RPM's for 3 & 1/2 hours non stop From Kenosha WI to Grand Haven MI. All the gauges, oil pressure, water temp, volts etc. were in check the whole trip they didn't move for basically 3-1/2 hours. We pull into a marina in Grand Haven for fuel and pump out, which took a good 45 minutes and as soon as I started moving down the river to our slip an alarm goes off and the port engine temp shows 200 degrees on the gauge. I shut the engine down and shot my temperature gun all over the engine and there wasn't a reading over 145 degrees. the gauge lowered to 160 we fired up the engine again and the alarm went off immediately and within seconds the gauge rose to 200 again. Water was flowing out the side exhaust as it should. Parked her in our slip and three hours later we still had a reading of 160 on the gauge. Is it a faulty sensor?, a loose wire? faulty gauge?