I have had a HeaterCraft heater in my 97 Ski Nautique and in my 2004 196. About every 5 years the heater cores start leaking. I simply remove the entire heater box, take it to my local radiator shop and they procure and install the core for me. Takes about 30 minutes to remove and install. Cost $50. Guy at the radiator shop says they fail due to bouncing around while going through chop. I really don't think it has anything to do with the water as my lake is very clean and doesn't have high iron content.
This method also gets the heater back in service within 24 hours.