Before you go too far down that rabbit hole, take the hose between the mixing elbow and the exchanger off, and check it for carbon build up. Also check in the elbow itself to see if there's build up, particularly whether there's build up where the hose from the exchanger ties in. Boats with diesels who are run infrequently, and at low rpms instead of cruising rpms, tend to get carbon build up in these areas, which can impede raw water flow through the mixing elbow.
I have a yanmar, but I *think* the mixing elbow was off the previous engine (a volvo) and I have to take that hose off and scrape around in the elbow with a screwdriver about once a year.
If you do get to the point where you're disassembling your heat exchanger, you can probably see fairly easily down the raw-water side of it for any buildup, once the cover's off. Do a visual inspection before your acid wash. Try the simple stuff first.
beej67, Checkered Past, 1980 32' Pearson 323, Panama City FL