A marine heat exchanger provides the cooling for a marine engine. In basic form it is a bundle of tubes where two cooling circuits meet and exchange the heat. This could be in the cooling circuit of an engine (marine heat exchanger) the oil circuit (oil cooler) or intake air (inter cooler).
For cooling water circuits, where possible, look for a combined marine heat exchanger manifold. When these are not available, then a separate marine heat exchanger may be the answer.
They are available in tubular form, or with an integral header tank, and models are available to suit engines of various outputs.