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, you'll never unlock the ability to dive, and I'm sure that'll make Esquie very sad. It's worth noting that this upgrade straight up isn't available until act three, so you might want to return then to find out how to get it.
How to unlock diving for Esquie in Clair Obscur: Expedition 33
To unlock the diving exploration ability, you need to reach relationship level 6 with Esquie. For those that don't know about relationship level, this increases by one each time you hang out with a character at camp, and there's usually a new hangout event whenever the game forces you to return there after a key story moment.
Considering six is the max relationship level, as mentioned, you won't be able to unlock diving until act three. If you've already gotten Esquie to relationship level 5 through previous hangout events, when you return to the map in act three, simply go to camp and speak to him to get the final one. Don't worry if you've missed previous hangouts; you can do them all in a row if you keep talking to him.
In this last hangout, Esquie realises that his neighbour, Francois, has been hiding the rock that gives him the ability to dive—rocks sink I guess? You'll head to Esquie's Nest, have a quick and easy fight with Francois, and after that Esquie will be able to investigate the glowing blue lights in the water that indicate a diving spot.
Each diving spot usually gives you five Colour of Lumina, but there is one diving spot in the final area that opens access to a special endgame area, so be sure to explore and try it out.