I'm glad you found the problem and resolved it.
You might consider pulling your heat exchanger core and cleaning it at some point. You can get a chemical cleaner or a radiator shop can dip it for you. It's usually not a very difficult job. Most marine diesel engine books will show you the steps. I like this one..
Amazon.com: Marine Diesel Engines: Maintenance and Repair Manual: Jean-luc Pallas: Books
It can certainly wait until the end of the season.