It isn't exactly salt-free. It's what is called "brackish" water, still with some salt in it. If there's a way to get rid of the salt other than boiling/collecting condensation, or using a special unit, I've never heard about it.
And even with the stagnant water thing, the water can still be contaminated, but the sand has filtered out the debris and algae. You could probably do the same thing by taking your shirt and putting it into a container (if you had one), putting a goodly amount of sand in the shirt, then pouring water onto the sand, letting it run through the sand and the shirt into the container. It would be filtered but not decontaminated, if there was a source of contamination.
(Yes, you could just use your shirt, but then it would have a large, green, smelly spot on it if you had to wear it again. <img src="/images/graemlins/frown.gif" alt="" /> )
Sue