Deep Sea Scale can be a valuable item, but you'll only need it for Clamperl in Pokémon Brilliant Diamond and Shining Pearl. After that, you use it to evolve Clamperl into Gorebyss. Here's what you need to know about how to find a Deep Sea Scale in Pokémon Brilliant Diamond and Brilliant Pearl.
We managed to find one while exploring the Grand Underground. You can make your way to the Grand Underground once you have an explorer kit, which you get from the old man near the Pokémon Center in Echoes.
While in the Grand Underground, you will have the chance to dig through fossil excavation sites. These excavation sites contain various rewards if you can remove them from the wall before the cave collapses, or you can unearth them before everything collapses. There is a lot of luck that has to do with finding a deep sea ladder this way.
When we found our deep sea ladder, we were in the central area of the Grand Underground. You can reach this location if you use the explorer kit while in Celestic Town, which you can reach after beating the fourth gym leader. As you progress through the story, you will gradually reach the city of Celestic.