water is fresh water. Im sure there is an easier means to make it drinkable than salt water.
There are a few things I know, if the water is still and crystal clear its likely acid since its killing everything and if you drink it, you likely die.
Murky water is filled with bacteria, since it aint running that likely just rain water that concentrated into a puddle, chances are you get dysentery and then you literally shit yourself to death.
What he was hoping to find was a spring, it doesnt seem to be one (hardly a surprise, it is a small island) and that water is useless, to decontaminate it they would need to boil it to kill off the bacteria so there is no difference from what they are doing.