anything made of rubber or rubber-like materials gradually degrade over time. Impellers are particularly prone to this. It is sometimes false economy to purchase impellers ...say on ebay, that have already been spares on someones boat for a long time. The impeller may look good but is very liable to "shed" a blade without warning. It is generally thought that about seven years is the shelf life of an impeller.

I once read of an engine distributor that had a number of new engines in old stock for some time - he finally sold them to a fleet operator who later discovered that every one of the water pumps on those engines leaked when put into service. There is a very fine lip on most water pump seals and these had partially bonded to the dry shafts and when started for the first time caused small bits to be torn from the lip.

We tend to sail in some pretty remote areas so I carry a fairly comprehensive selection of parts including fresh and raw water pumps, all of which I have rebuilt myself. When doing so, I make certain to lubricate the seals with SYNTHETIC grease before assembly, this helps seal the seal so to speak and prevent it bonding with mating surfaces. Water pump impellers can benefit from the same treatment when installing.

Our spare belts are all Gates (which I find will outlast Yanmars by a good margin) I vacuum seal them in plastic bags for storage aboard. High quality belts do seem to outlast most seals and other rubber parts when in storage.