5x per year for 2 years is really low usage. Since you kept the tank full, you may have retained some really old gas in there, and even with the tank full, the various compounds in the gas will react chemically over time and go sour/stale/bad. Some of these compounds may have a lower flash point than normal, and so you get detonation.

In the future, run the tank close to empty at least once every year, like in the spring and in the fall, then over the winter keep it topped off and add stabilizer. If you just don't run the boat enough to do this, drain the tank into your truck every spring and fall, and refill with fresh fuel.

BTW, a fuel/water separator doesn't help matters if the fuel has broken down due to age.

Regards, and sorry about your repair bill

Finegrain in Woodinville