It would require the
engine heater exchanger and the
calorifier to both have failed which on a 4 year old
boat is highly unlikely.
However, did they connect the hoses up correctly when the
boat was built or did the manage to get the hoses all mixed up so saltwater is going to the
calorifier?
Certainly something odd., how about
buying a TDS meter to check it is
salt and not
antifreeze you can taste. At £22 it won't break the bank.
https://www.amazon.co.uk/HM-Digital-...ved+salt+meter