The ship has to return, empty or not. If the shipping company can recoup some of those costs (labor, fuel, insurance, etc.) by selling the otherwise empty space to someone who sells bottled island water, it's a net gain for both the shipping company and the Fijian economy.

Of course it's good for the shipping company and the water sellers. Otherwise they wouldn't do it.

The point, which you seem to be purposefully missing, is it's a net loss for the world.

Unless the shipped amount is exactly the ballast... But even then the ballast and any cargo is placed differently so even that point wouldn't work.