Make sure your cooling lines aren't kinked or clogged and your water pickup is low enough. Run your engine slightly rich. Only run it at low speed out of the water and get it in the water immediately after starting. I always ran helicopter fuel with a high oil content in my boats and even added a couple of ounces of castor oil per gallon. Also make sure you flush your engine with after run oil after each use. If your going through engines like that you are most likely over heating them. O.S engines are great engines and with proper care should last many years.
The mistake that most new boaters make is running the engines too lean. When you fire it up you want a slight lag when hitting the throttle. When the boat hits the water, the load of the prop will lean it out.
Make sure that you use fuel made for boats, not planes or cars as it doesn't have enough il content. You want 18-20% oil. Don't be afraid to run high nitro either as most of us run 40-65%.