Yes and no

My understanding is that you generally only get a boil warning when there's a material risk that the sewage lines might have contaminated the water supply.

Since I take it on the authority of history that dysentery is not something to mess around with, I am inclined to take those sorts of things seriously and flush plumbing used for potable water. That said, I don't water hot from the tap, so unless you do I don't know why you would need to empty the hot water heater.

I would place garden hoses in an entirely different category. The risks attendant to drinking from a garden hose are remote.