If your impeller sits in one position for a long time, there is a good chance it will fail the next season. You don't want it to fail when you are underway, even if you carry a replacement! Think of where all the pieces of plastic/rubber will go and get stuck. This is why most engine manufacturers recommend they be taken out and replaced every year!
The water from many sources is very hard (full of minerals) and waiting to solidify in your engine's block, hoses and fittings! Think about what happens to a hot-water tank over time. Even if you do not drain it, a flush with clean, soft water will help. Add some environmental anti-freeze and your engine will thank you.
Fuel needs to be stabilized or drained, especially from carburetors. Your injectors will thank you for some stabilizer or cleaner.
Put it in cave storage, but take 30 minutes to do the above.