What annual service covers
Engine oil and filter where the engine takes them, gear lube out and inspected, spark plugs, fuel filter and water separator, the water pump impeller on its schedule, anodes checked and replaced before they are gone rather than after, belts and hoses, battery condition and terminals, steering and control cables, bilge pump and float switch, and the safety gear that the Marine Patrol will ask about.
Why anodes matter more than owners expect
Anodes are sacrificial by design: they corrode so that the drive, the shaft and the fasteners do not. An anode that is half gone is doing its job. An anode that is entirely gone stopped doing its job at some point you did not see, and everything downstream of it has been unprotected since.
Mid-season checks
A boat used most weekends between ice-out and October is worth a look partway through. Fluid levels, the impeller tell-tale, belt condition and a battery check take very little time and catch the failures that otherwise happen on the water.
What annual service actually prevents
The argument for annual service is not tidiness, it is arithmetic. An impeller and gear lube is a small parts bill and an afternoon. An overheat that was allowed to continue is a powerhead. Water past a lower unit seal that went unnoticed for a season is a gearset. In each case the preventive job costs a fraction of the failure it stops.
It also changes what a breakdown looks like. A boat serviced every year tends to fail in predictable, cheap ways, usually at the dock. A boat that has not been serviced in four years fails in expensive ways, usually a long way from one.
The other thing an annual service buys is a record. Knowing when the impeller last went in, when the gear lube was last clean and what the anodes looked like in the autumn turns a future diagnosis from guesswork into a short conversation — and it is worth real money when the boat is eventually sold.