Fiji imports more goods than it exports. As such, ships that bring goods to the islands come in full but leave without a full load. Carrying water, while it seems silly, is a use of that empty capacity.

Ships spend more fuel the more weight they ship. If you ship something unnecessary, you are wasting fuel for nothing.

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.