I bought a "Nomad H-2-0" 12V power sprayer from amazon.com for $35.00 + shipping. it holds 3 gal of water and will spray the contents for about 8 minutes -which is not using a lot of water compared to a transom shower.
Its a good alternative to cutting a hole, running lines and mounting a cockpit shower. Its about the size of a 5gal jerry can and can be filled with warm water. It has adjustable spray nozzle and will sub as a washdown on deck with its 20' hose, 10" DC cord.