Having worked on and plumbed boats, the bigger issue is actually growth and especially mineral deposits. Corrosion is less of an issue since most plumbing is actually plastic at this point. Although any water that goes into an appliance needs to be fresh water, so it would really ONLY be for showering.

Sewage in particular will create hard deposits in plumbing that needs to be dealt with every few years at a minimum.

Frankly, unless you are in a rather extreme environment, like a desert, or a boat where you have to carry or make all your own fresh water, saving a few gallons on showering and washing is pretty inefficient. You could have a far larger impact by changing habits, and ensuring low flow appliances.