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If you didn't have a thermostat in there to hold the water in the engine to raise it to operating temperature it would run too cold. Just like your carwould in the winter.
The reason for a thermostat is to keep the engine warm enough to support good combustion. On a car it is the radiators job to cool the engines few gallons of coolant. With most boats you have got a endless supply of cool water for the engine. I have never heard of a lake boiling over because of a boat overheating.
