A hot bath or shower is one of the best inventions ever. Nothing relaxes me more than a nice hot shower at the end of the day. So when shit hits the fan what are you gonna do? Sure if you have gas you'll still have a water heater but for how long. What about remote locations where gas is hard to come by or there's no electricity?
I know about the solar hot showers and I also know you could always take cold showers I did it for nine months and ain't doing it again. I've got two solutions for this dilemma.
Old steel milk can
I've personally done this and can assure you it is quick easy hot water. Get a nice bed of coals going then put the milk can on the coals. Fill it up, wait about 15 minutes, Done. The bottom of the can and the handles will probably be hot so be careful and use leather gloves or a towel. I pour mine into a bathtub that's in a shed by the fire pit. One milk can is plenty of water for a nice hot bath.
Gas water heater converted to wood burning
I have not tried this yet but it seems to be the same principal as my milk can. Get a used gas water heater and remove the guts. Next you need a firebox. The author used an old rim with the center cut out. Finally make a washer that will be the diameter of the water heater. Start a fire in the firebox and according to their website you'll have hot water in about 15 minutes. Here's the link to the article.