This is a Reshade Preset, which changes Cat Quest 2 to have a slighlty less saturated and more epic look. Because of that it also makes Caves and Water look better.

/// Installation ///
(If you already know how to install Reshades then you can ignore this)

Step 1: Install Reshade:

To install this Reshade install Reshade from the Official Reshade Website, Run the Installer and select Cat Quest 2. 
Then select the Rendering API Direct3D 10/11/12

When you launch the Game you should see this Message Popping up in the upper left corner:

If the Keybind on your Message is different, then don´t worry. Reshade is working. I just have it set to a different Keybind.
If you dont´t see this menu , when you launch the Game then Reshade most likely isn´t working.
If that is the case, then try to reinstall it with the Installer, and make sure you are using the correct rendering API.

Step 2: Install the Reshade Preset
Unzip the Folder of the Reshade Preset that you want to use (In this Case the Shade of Blue Preset) with Winrar,
and put it in the Cat Quest 2 directory.

Step 3: Apply the Reshade

Almost Done!
Now you just have to Launch the Game and use the Keybind to open the Reshade menu, which you can see in the Message  in the Top left Corner when you launch the Game.

Now you just have to select the Preset in the Reshade Menu you want to use and you are done!