Looks interesing but (at least upon initial read) somewhat conflicting with the following statements all on the first page (emboldening mine):
RinseKit is the only portable shower to have the pressure of a garden hose
without pumping or
batteries.
Now with your help, very soon it will also be the ONLY pressurized shower to
have a battery-powered heater system that quickly and safely heats water to 100°F
Rechargeable
battery quickly heats the water in the RinseKit chamber to 100ºF
Battery charges from the car or at home
I guess what they are saying is that it doesn't have a battery to provide pressure, but does have a battery to heat the water. On the other hand, this relies upon filling from a pressurized hose and oftentimes when I'm out I'd have non- or less-pressurized water (fill from lake, water jug, hand pump, slow-flowing hose, etc.).
Still interesting, and never hurts to have additional choices. I could see this being handy around the house or other places you can fill with a high pressure hose (but maybe can't reach with your garden hose or want warm water).
If hooking up to 12-volts is acceptable, there is the Big Kahuna family (basically a bilge pump in a container with a wand).
https://bigkahunashowers.com/