Those symptoms sound exactly like leaking valve guides. First off, bluish smoke is oil. Black smoke is unburnt fuel
. So you have an oil burning situation. That it didn't happen noticeably until after the engine had sat for a couple hours, tells me that oil had dripped down into the cylinders from the valve guides.
When an engine gets hot, as yours was after being used, oil is thinner and valve guides leak more. They are probably leaking a small amount all the time but not enough to produce real blue smoke. And small enough to not change your oil level.
The good news is that leaking valve guides are super common on older engines and need not require any action for quite some time. They do not themselves affect the engine performance, so long as you keep the oil topped off. Often, by the time the valve guides are going there are other things wearing out that may point to a rebuild or replacement, but you sound like she is running fine otherwise.
I once ran a VW for thousands of miles with leaking valve guides doing exactly like your boat, with no ill effects except for the environmental air pollution at start up.