First step is making sure you have full flow of water between the through hull and the perko filter. Often barnacles, kelp, plastic can restrict water flow. Liquid plumber won't help...but often a dive overboard with a long screwdriver and/or coathanger can dislodge whatever is restricting the flow.
If that is not the problem...and you think the impeller is good...check the flow by removing the connecting hose for a few seconds with the engine running. If that check out...it is on to the heat exchanger I'm afraid but it is NOT that difficult a job. Alternatively to removing the exchanger if you are pretty sure the blockage is NOT a piece of impeller...is to use muriatic acid solution at the engine intake and let it run through until you see it coming out the exhaust...let it sit for a few minutes and then flush it thorugh by running the engine normally. That will often clear the pipes. Otherwise they need to be reamed out physically.