You already mentioned the usual suspects. Now is the time to be more thorough.
1) What are the directions on Canadian Tire's diesel antifreeze? Full strength or diluted?
2) What does your engine specify for coolant? Does Canadian Tire's product fill the bill?
3) Is the impeller you installed an OEM part for that engine? Heat is an engine's worst enemy. Overheating shortens the life of a diesel engine and we all know how expensive they are to replace. This is not a good place to save a few bucks.
4) When you say the raw water system is clear, do you mean you checked the entire path from the raw water intake through-hull through the mixing elbow? Let's go through the steps:
-- Did you run a screwdriver or pipe up the intake through-hull to ensure there's no obstruction, such as barnacle growth?
-- Did you ensure the strainer is clear?
-- Did you clean the heat exchanger? You'll need to do that every few years. I did it when I changed the fresh water coolant.
-- Did you examine the mixing elbow and clean out the crud?
If you don't want to go under the boat to check the through-hull, you can short-cut by closing the seacock, taking the engine's raw water hose off the strainer, then opening the seacock for a short time to assess the flow. Assuming your strainer output is well below the water line
, the flow you see there should be greater than the flow you see out of the water muffler. If it isn't you found your choke point. If you don't want the water running into the bilge, I'm sure you can put a temporary hose on it and run it to a bucket.
If the strainer is vertically mounted as mine is, the exit is near the water line
and the lack of pressure can be deceiving. You can take the strainer off and do the same test with just the seacock, but then you need go to the trouble of getting a good seal on the threads when you reseat the strainer.
Good luck. Hope this helps!