Maybe. If the mains water supply gets contaminated (but is still flowing), the contamination may make it into your hot water tank if you don't disconnect it quick enough.

You also need to make sure you have the where-withal to get the water out of the tank. In my system, the hot water tap taps the tank at the top, where the water is hottest, which works because it's under pressure from the cold water tank in the roof. When the cold water tank is empty, the hot water taps won't have pressure. There must be a means to drain the hot water tank from the bottom, but it probably involves tools and extra piping. I'm rubbish at DIY, so it'd be naive for me to assume I could manage it.