Deepsea Scale was introduced when Cataclysm was released, so it can mainly be found by skinning high level creatures in Kalimdor or The Eastern Kingdom and even in Pandaria. Deepsea Scales can be obtained by skinning aquatic cretures like giant fish or sharks. The best place to farm Deepsea Scale would have to be Shimmering Expanse as the zone is mostly submerged in water. Creatures that you’re looking for would be Anemone Frenzy, Barbfin Skimmer, Giant Driftray, Oceanic Broadhead Ravenous Oceanic Broadhead, Snapjaw Grouper, Spiketooth Eel, Slickback Remora, Spotted Swellfish, and Sand Ray.
See below for areas to farm Deepsea Scale in WoW.