In 1899 Otto Hensel invented an oscillating tub that could give a vigorous bath while conserving water:

The essential object of this invention is to provide a tub that will fill a long-felt want in hospitals, sanitariums, and other institutions, as well as in private residences, which will by a simple rocking motion agitate and throw the water with more or less violence against the body of the person in the tub.

In Sylvie and Bruno Lewis Carroll goes this one better with the Active Tourist’s Portable Bath, a bag in which one can bathe in half a gallon of water:

“The A.T. hangs up the P.B. on a nail — thus. He then empties the water jug into it — places the empty jug below the bag — leaps into the air — descends head-first into the bag — the water rises round him to the top of the bag — and there you are! The A.T. is as much under water as if he’d gone a mile or two down into the Atlantic!”