Is there a way to get hot water without altering plumbing or electrical wiring too much?
Crystallization of a supersaturated solution of sodium acetate will release heat - once. But then it is just a solid. It works nicely for pocket warmers, but since you have to heat the flexible container in boiling water prior to re-use, this and other comparable processes are of little use for a water supply.

At the end of the day, you won't get around plumbing to tap an energy source.

Your options would be:


solarthermal
geothermal
heat released from the fermentation of organic waste