Heat exchangers perform much the same function as a radiator in a car - removing heat from the coolant which circulates through the engine.  Instead of a fan driving air over a radiator, raw water (water a vessel is floating in) flows through one side of a heat exchanger while coolant flows through the other side.

Heat exchangers - also called "oil coolers" if the connected to the turbocharger or transmission fluid circuit - are an essential part of indirect cooled marine diesel engines.  Tubes need to be kept clean of scale build-up and pieces of impeller vanes and broken up anodes.