They prevent water rushing up your exhaust and into the engine. Depending
on the type, replacing them usually is a pretty simple task unless you have
to replace the whole assembly, including the exhaust pipe. That involves
removing and replacing the portion of the exhaust pipe that goes through the
transom.

jps

my boat has an OMC (Ford) King Cobra I/O 351(?) Does that help
anyone to know if it is a simple job. Should I have this done when I
take the boat out for the season?
thanks