I have purchased the gear hammocks I have used at West Marine, but I am sure that there are cheaper ones.
We have a variety of "Sun Showers." I believe that the smallest one is about 1/2 gallon. It is enough for a one person rinse off after the soap up. . I have taken a number of one gallon of water or less showers. Our "hang up" Sun Shower is the 2.5 gallon one. We also have a 5 gallon (?) which has a mesh bag, and bladder, plus foot pump, so you can leave the bag on deck, and then use the long hose under pressure to rinse off with.