A little more info on your setup would be helpful. What year, size engine, and type of cooling system do you have?
When you say "thermostat is not reading right", are you referring to the temp gauge? If so, how do you know the gauge is incorrect?
Do you have a laser temp gun? They are really handy and will help you find the source of your problem quickly.
Is this a new to you boat? Are you sure there is a thermostat installed?
Sorry for the questions, just want to help you figure this out.
2006 Bayliner BR205
A1 Gen 2 1.47
5.0 carb engine