I believe the most common reason for lack of water flow is a clogged heat exchanger.
I've had the following happen to me before:
Mud in the filter
Grass in the thru hull fitting
Pipe collapsing under pressure load because it wasn't the right type of pipe
Broken impeller blades
Carbon gunk in the mixing elbow
An entire family of barnacles relocated to the interior of a pipe right before the filter housing.
I'd say I've seen a lot of problems with my raw water system, and I've done a lot of repairs and replacements in it, but I've never had a problem with my heat exchanger. In my experience, the heat exchanger itself is the least likely thing to screw up.
The only time I'd be worried about the heat exchanger is if you lost a bunch of impeller blades. It doesn't hurt to check, though. Basically, just disconnect stuff until you narrow the problem down, as I mentioned above.
I have a Yanmar 3GM30F freshwater/seawater cooled engine, which doesn't need/use zincs