As far as impellors and belts go for a broads based boat I am comfortable with an annual condition check and aslong as the parts still look serviceable and Im carrying spares I am comfortable with that.
I dont see the sense in changing parts that are still perfectly serviceable. Belts on a boat should last a long time, I changed my last set at 6 years and apart from slight wear on the friction surface they were otherwise fine!
But, I would always change oil, oil filter and fuel filters annually and gearbox oil every few years also.