In the manual for my 3GM30, I believe there is a water flow graphic. Maybe you can look at the one for your engine, look at the path of the water to see where there may be some clogging. The head gasket area would be my guess. Is it possible that someone put a head gasket on upside down or reversed in the past and blocked some ports? If you could isolate the head and put a LITTLE flow through it with a hose, maybe it would tell you if it was blocked. Be careful putting pressure with a hose as it can ruin the head gasket seal and get water in the cylinders. It's usually not recommended to do that but if you're going to pull the head off anyway, maybe you can try it. How many hours are on this motor? How long has it had salt water running through it? Cast iron becomes crystallized by salt water after many years and at some point actually becomes porous, especially if the engine is not flushed between uses with fresh water. Your engine is probably not old enough for that but it's something to consider. http://www.oldmarineengine.com/techn...rrosion_1.html
By the way, your 3600 RPM is quite high if you're running at that RPM continually. Most of these reach hull speed at around 2800 so revving up any higher is pushing the limit.