The Sinking City 2 trident is part of a puzzle in the Church area that involves the nearby obelisk. It's not very clear though, and even if you find and read the various notes relating to it, it can be unclear what you're meant to do with any of it.
I've solved it all already and have explained the process fully in my in-depth Sinking City 2 walkthrough covering the flooded city section. However, if you just want the short version, here it is.
