How to unlock diving in Clair Obscur: Expedition 33

Learning how to dive with Esquie in Clair Obscur: Expedition 33 is the final exploration upgrade you'll get access to, but unlike the others, this ability is entirely optional. While out swimming, you've likely noticed glowing blue lights in the water, but when you try to interact with them you simply get a message saying "Esquie can't go underwater yet".

"Yet" is kind of a funny phrasing when you consider that this upgrade doesn't come from the main story—if you don't do what I'm about to describe below...