 Which thermostat ???????
Marine thermostats are lower temperature than automotive thermostats for several reasons...
1. Marine engines run much harder. When is the last time you ran a Chevy 5.0 at 3000+ rpm in a highway vehicle, with a heavy load, for extended periods? That would be like towing in second gear up a hill at highway speeds all day! Well, cruising in your boat is just like that!
2. Salt or mineral accumulation. Gotta keep those temperatures down or else you will start a crystal growing factory inside the block and exhaust manifolds. Higher temperatures = higher rates of chemical reaction with metal (corrosion).
3. Engine compartment temperatures. Gotta keep those temperatures from getting too high or there could be safety considerations. Remember, the water has to cool the exhaust manifolds and exhaust too!
4. Performance. The cooler the air going into your intake, the more horsepower you can maintain. Unlike a car, there is not a whole lot of air circulation in your engine compartment.
5. Engine accessory life. Remember those engine compartment temperatures? Anything that can keep the temperature down will increase the life of wiring, belts, alternator, and so forth.
6. Outdrive temperatures. Gotta cool that exhaust or excessive temperatures going through the drive leg could significantly reduce the life of seals, boots, etc.
I think that covers it...
By the way, my raw water cooled Mercruiser uses a 143 degree thermostat on a Ford small block V-8. The motor is 28 years old and is just now starting to burn a little oil! | 
