Saltwater Trouble

It appears saltwater has somehow got into my motor. The boat is a28'
twin.Last summer it over heated, we waited till it cooled off,added water
and then went back to the finger and tied up.It was used sparingly but
without incident,then came Juan.I was changing thermostats this spring when
it became evident there was a problem.The anti freeze had turned to
toothpaste coloured crysallized substance.We used a product called Salt X
which has certainly helped.The motor has a closed fresh water system so
after doing a visual check,I took the expansion chamber off and had it
tested.It was ok, but now I'm lost.The motor when last used worked fine.No
miss or skips and no increased motor noise from the wet exhaust system.I
would be most gratefull for your opinions and directions..........Thanks