I thought they were a useless piece of cruiser junk.
But they do work very well - too well, in fact - in the tropics.
Mine is on deck on the shady side and the rose is through the bathroom window (Thats the Heads hatch for the old fogies). Its too hot to use in the middle of the day, but great at 4pm ready for Happy Hour.
I have some Blu Tack plugging up some of the holes in the rose to reduce the water flow.
I would like a better tap system on it because soapy hands make it difficult.
In the higher latitudes one would think it would still work viably if the shower was had at the right time.
In all, the solar showers do work in reducing water consumption
and reducing power to heat water. At their price
it would not be a vast investment if it didn't work in some country frozen solid.... like the UK