While Jack and I seldom agree, post #3 offers you some good info. Take heed!
If OEM, your exhaust system does not include "risers/spacers"...... the 888 has log style manifolds and rear elbows.
(I've owned and have worked on many of these)
In either event, warm the engine up and crack all exhaust component bolts loose.
When things cool back down, remove all exhaust components.
Check the seawater transfer ports between manifolds and elbows.
These must all be clean and free-flowing.
You enquired about the area where the seawater enters the exhaust flow. That would be the "mixing" chamber.
Yes..... very important that these ports are clear and free-flowing as well.
All of this can be examined with the exhaust system removed.
The belt driven pump at the front of the Ford Engine is the "engine circulating pump".
For a Closed Cooling system engine, the Automotive circ pump works just fine.
However, for Raw Water cooled engines, the metallic impeller must be made of bronze or SS in order to hold up in that invironment.
If the metallic impeller was not the correct material for Raw Water use, it may be bad.
I would suggest that you pull it and examine it.
Yes, this occurs elsewhere within the exhaust Elbows.
In my opinion, the OEM 888 exhaust cooling system is more complex than need be.
If you were to replace the exhaust manifolds/elbows, you can simplify the system by using a "center-rise" manifold and elbow, and going with a more conventional T-stat housing.
You would supply cooling water to the manifolds only, allowing the transfer ports to direct it up