On a deisel motor in a boat why does it have coolant and a water pump?

I just got a new boat with a diesel engine and I don't understand why there is a coolant tank as well as a water pump? I think the water pump goes to the heat exchanger but what actually cools the motor - how does this stuff work?On a deisel motor in a boat why does it have coolant and a water pump?typical engines the water is pumped into the header and cools the engine off and then flows to the radiator to cool the water off. the water pump just keeps it moving.