I’m not sure what region you live in or what the insulation is like in your attic but if your attic gets hot enough you may not need much to heat your water. Here in Texas summer temps in an attic can reach 140. That’s plenty hot for bathing or what have you. You mentioned a fireplace. Is it a solid brick chimney or does it have a stovepipe insert? If it’s an insert you could wrap the stovepipe with some copper tubing and use that as your circulation loop. That way you would be able to heat water while you heat your home in the winter or if electricity is out. You could circulate the water with a 12 volt pump run on a car battery with a solar charger. If you have a insulated attic you could just spool a couple hundred feet of water hose on your roof durring the summer and use that as your loop. Just trying to help with some A/C free options.