This item drops from the diseased shark in Erudin's harbor. The diseased shark can also drop other items which are also called "shark skin" but are totally different items from this one.
The shark skin with the "Heretic Shark Skin" lore is used for the Barnacle Breastplate quest. It only drops from the diseased shark.
The shark skin that does not have the lore tag can drop from pretty much any shark, including the diseased shark, and is not used in the Barnacle Breastplate quest.