I have used this sort of shower at various festivals and while camping, keeping the tail end in a bucket of water. It is very good for washing hair and face! What you do is you focus on washing your face first, then your hair and you keep yourself over a second bucket to retain the waste water. Now, since the waste from washing your face and hair isn't very bad, you can use the rest to wash your body, simply by switching buckets. You just put a cloth into the water, wash your body that way by scrubbing and squeezing the cloth out back into the first bucket, then use the rest to rinse off the soap and grime.

I actually don't use the shower head at all once I am done with washing my hair and face, as it is less convenient than a nice rag. I end up using biodegradable peppermint soap and just wipe it all off with a damp rag I keep squeezing into the bucket, and then towel dry. However you could shower rinse off the body soap with the waste water from washing your hair and it works fine.

You can easily patch in an extension and power it off of 12v mains ;-)

I recently bought a Rule IL200 submersible inline pump for under GBP 20. It runs from 12V and sits on the end of a hose. It's been very handy for bailing out the dinghies at my sailing club, as an auxillary bilge pump, and I've used it for a sort-of shower once. A shower head with a hose attachment would turn it into a proper shower. It draws about 1-2A.

One item, many uses!

I came across this rather natty alternative to a solar shower.  All the advantages of a pressurised shower with none of the drawbacks!  Well, not quite perhaps, because it only runs for a minute, in which time it uses 2l.  Maybe I'll stick with my bird bath.

However, similar items are already made that plug into the 12v.  These are a bit greedy on your water, but still a lot simpler than all the plumbing required for an onboard shower.  Certainly, you can't complain about the cost, although I have no idea how long they'd last.  If I were a cleaner person, I might consider one!