Figured Id add to this.
Every year I spend a couple of weeks on the two islands off the Langebaan coast - the Pelican watch Conservation project. Theres no running water, no electricity in fact no taps on the island which Is fun!
Essentially you’re marooned there for 7 nights (its awesome)
For showers we have a variety of bags that we’ve used since 2012 and honestly the only one that lasts is the OXtrail one. So I recommend paying a little bit more for that if its in you’re personal capacity.
Also dont fill it up - 3/4 at the most. And a last tip is you can pour in a litre of boiling water if youve had an overcast day. That said even on a sunny day getting the bag out the wind warms it up much quicker, they cool down really quickly in the wind.