Your marine thermostat is set for a lower operating temperature than a car. You are also right that a higher operating temperature would get better performance out of your engine. However, swapping thermostats may not be as simple as it sounds.

When raw sea or lake water is heated above 145 degrees, deposits build up inside the cooling water passages of the engine. The problem is worse in salt water, but high engine temperature can also pull the dissolved minerals out of lake water. Deposits in cooling passages cause “hot spots” that lead to premature engine failure.

It may be possible to cram a high-temperature automotive thermostat into a marine engine based on a block used in cars. This doesn’t make it a good idea. An automobile thermostat may not allow enough bypass water to flow through the full system. Although the engine may be fine, a lack of cooling water could cause overheating elsewhere, such as the exhaust manifolds.

While expensive, the best solution is an aftermarket closed cooling system. This eliminates raw water from the engine in favor of nontoxic coolant that can run at a more efficient temperature. Raw sea or lake water goes only as far as a heat exchanger where heat is taken from the engine coolant. An exchanger performs the same function in a boat as the radiator in a car.

Aftermarket closed cooling systems for marine engines are available from several manufacturers. One size does not fit all. It is necessary to match the specific make and model of engine with the correct kit.