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.