But water isn't currency. Currency is a substitute for a thing of value, an symbol of exchange used in circumstances where direct barter is inconvenient or unfeasible. Water is an actual commodity, and trading water for other stuff is simple barter, not a monetary exchange.
Never mind, of course, that the idea of a starfaring power lacking for water is completely nonsensical. Water is one of the most abundant substances in the universe
. There are thousands of times more water in the comets and moons of the outer Solar System than there is on the entire planet Earth. Heck, the only reason there is
water on Earth is because a small fraction of those comets crashed into it over billions of years. So the Kazon lacking water is as crazy as... oh... Voyager
being unable to find deuterium.