If your boat has a strainer it will be between the hull water intake and the trans cooler . On the suction side of the raw water pump it is possible to draw in air if any of the hoses or seals are leaking. Check the O ring on the raw water pump. If you can get a Infrared Temperature Gun point it at the thermostat housing it will give you a pretty close reading of the engine temp . It is possible the gauge is off.