Abstract: A marine power system having closed loop cooling includes a marine engine having a cooling fluid passage defined therethrough through which a cooling fluid stream may pass. A shell and tube heat exchanger has a tube side flow path and a shell side flow path defined therein. Cooling fluid conduits connect the cooling fluid passage from the marine engine to the tube side flow path so that the cooling fluid stream from the engine is directed through the tube side flow path of the heat exchanger. A raw water supply system directs a raw water stream from a body of water through the shell side flow path and then back to the body of water. The heat exchanger includes an outer housing and a tube bundle receiver in the outer housing. The outer housing is comprised of a shell and first and second end caps. The tube bundle includes a plurality of straight parallel tubes held between two spaced bundle bases. The housing and the bundle bases are constructed of non-metallic corrosion resistant materials.