if mechanic "don't know how to fix" time to get new, competent mechanic....
if a vane was missing, it could obstruct the water flow (=overheat)....if the elbow clogs (water outlet where water gets 'dumped into' exhaust), reduced water flow = overheat....similarly, if the exhaust baffle fails (usually due to lack of cooling water) if can constrict the exhaust and impact engine performance...
Routine replacement of the exhaust is normal in any water other than very low mineral fresh water lakes.... most of the creeks on the chesapeake offer an environment that yields ~ 7 years of normal service life, with typical usage....you may be able to clean out the elbow once or twice but you won't see much service life after that.