Are you referring to the bleed from the anti-siphon’ at top of coolant pipes, see attached ? On my 331 there is a constant stream from this to a hull outlet on the portside. and it has been like this for the last 10 years. The actual exhaust with coolant seems to be discharged in bursts, perhaps your passerby was referring to that ?
If the exhaust need cleaning surely there would be noticeable reduction in power ?
Have you tried draining the muffer ? On the 331 the muffler is a fibreglass box with a drain plug on the side which I have periodically undone to see if there is any blockage, so far only clear water has come out.