Jellyfish Necklace

It gives off a faint glow all the time and brightens sharply the moment you are underwater. Anything stronger on your head drowns it out, so a mining helmet and this necklace worn together means you will never see it doing anything. It also goes dark in a vanity slot, since the glow only comes from the functional one.

Rarely on its own. The reason to hold on to one is the diving gear it feeds into, which folds the light in with underwater breathing and makes the ocean floor genuinely workable. Until you can build that, it is a novelty.

Statue spawned jellyfish still drop it, and the sell price is high enough that a jellyfish statue makes a passable hands off coin generator. Current versions cut the drop rate from statue spawns specifically, so treat it as something running in the background rather than a fast farm.