It sounds like you have raw water
getting into the fresh water, Probably a gasket
in the heat exchanger gone , hopefully not a cracked tube. Your overflow is only part of the fresh water expansion system. If the expansion tank overfills it means you are adding water from an external source (raw water intake system) The most common point where those two systems are in close proxiemety is the heat exchangers. You might have one or two of these on the engine
depending on the type of engine. Good luck. Pressure test the heat exchanger.