Short answer
Change the oil and filter before the lay-up, not after, so the engine does not sit in contaminated oil. Fill the fuel tank to limit condensation, check coolant condition and concentration, protect the raw-water circuit as your manual specifies, and keep batteries maintained rather than simply disconnected.
You will need: Service consumables, fuel treatment as directed, battery maintenance, covers
What damages an engine in storage is not the storage. It is being put away dirty, wet and half-fuelled.
Before it sits
- Change the oil and filter. Used oil holds combustion acids and moisture that attack bearing surfaces over months. This is the single highest-value step.
- Fill the fuel tank to reduce the air space that produces condensation, and treat as your manual and fuel supplier direct.
- Check coolant condition and concentration. Inhibitor degrades on a calendar.
- Protect the raw-water circuit as your manual specifies for your climate and duration.
- Batteries: maintain rather than abandon. A battery left to self-discharge over months may not come back.
- Close seacocks, note that you have, and leave a reminder at the panel.
Australian conditions, specifically
The tropical wet season (roughly November to April in the north) is the harshest lay-up environment in the country: heat, humidity and months of disuse are exactly the conditions in which water and microbial growth take over a fuel tank. See Darwin and Cairns.
A southern winter is gentler but longer, and freezing is a genuine consideration in parts of Tasmania and the alpine inland — coolant concentration should be correct rather than approximate. See Hobart.
Coming back
Do not simply turn the key. Check oil and coolant, inspect the raw-water circuit and impeller, look for water in the fuel, check belts and hoses, confirm seacocks and strainer, and watch carefully for exhaust water on the first start. A boat woken from a wet season needs the fuel checked before it needs anything else.