Do the water passages get clogged after years of use and if the
engine is under strain it will show up as overheating?
ANSWER: A MOST DEFINITE --- YES!

Your engine is probably 'salted': both in the exhaust manifold and the
internals of the engine proper. Especially if the thermostat opens at
above 190 degrees the salting process is amplified.
Sea water when heated above 150 degrees will precipitate carbonates
(salts) which CONTINUOUSLY form an insoluble coating on the engine
internals. This fouling will retard heat transfer and if the 'salt'
deposition is significant will block the flow of cooling water.

Best remedy is to drain the cooling water, then add MARSOLVE
(www.marsolve.com) circulate* this through and let sit to dissolve the
'salt' accumulations. Marsolve is commercial descaling compound that is
biodegradable and will NOT dissolve the base metal of the engine. Many
use muriatic acid to 'pickle' marine engine internals but the muriatic
will also react with or dissolve engine base metal. ALL salt water
cooled engines should be so 'pickled' every few years to remove the fouling.

Once the engine is 'pickled' make a decision:
1. Install a thermostat that opens fully at 135 degrees and suffer less
thermodynamic efficiency because the combustion chambers operate at a
less combustion-efficient state. OR
2. Plan to 'pickle' the engine every few years.

*circulate = pull the inlet water hose from the closed through-hull.
Remove the hose from the water injection port to the heat/steam rise
elbow. Put the inlet water hose, put a temporary hose from the muffler
water injection into a large bucket, pour the marsolve into the bucket
and run the engine until the marsolve fills the engine, Keep running
until the marsolve solution is warm, then shut down the engine and let
the marsolve soak. Etc.