There's more to the ocean than just salt water. Like poo and urine and other wastes that marine life excretes.
So I think that Japan has formulated a plastic bottle (if it IS truly of plastic material) that dissolves in dirty, salty, alkaline-water (because ocean water is an alkaline (at ~8.1 on the pH scale) but drinkable water isn't (at ~7 on the pH scale - which is considered neutral)) and not for drinkable sink, well, or filtered water, but there could be problems with it though, like faster deterioration (I think that, well, it dissolves in water, duh!) :/