In order for the water to cool the engine it must flow in the water jackets that surround the cylinder bores. If these passages are blocked with sand, mud or salt build up the water will not be able to flow around these passages or effectively conduct heat away from the surrounding metal. This video experiments with various solutions for flushing your engine to clean this passages. If cleaning with acid it is recommended that you flush afterwards with sodium bicarbonate to neutralise any remaining acid.