If a leak is detected between the raw water and freshwater systems, it is most likely to be in the heat exchanger. The seals between the two systems may need to be renewed, tightened etc. This is a water system, so jointing compound or gaskets can be used with no problems.
A leak will be apparent if water drips from the freshwater overflow pipe when the engine is running. This pipe is usually fitted to the side of the filler cap on the header tank, and exits under the engine. The pipe can be removed from the cap to check for any leaks.