If you already have a wood burning stove at home that heats your water, this is not going to be of much interest to you. If however you rely on gas/electricity to heat your water, this is a great little emergency preparedness project for back-up hot water in emergencies. It would also be useful for remote/0ff-the-grid cabins etc.
You can pick up those old water heaters for nothing, or next to nothing from FreeCycle and Craig’s list etc. They often get wasted or recycled when people get their heating systems replaced for new.