You CAN use Unity's Shader Graph to create custom shaders that work with Unity's Built-in Render Pipeline.
Check out some of these articles in the STYLY Magazine:
That said, you cannot upload the shader graph you create using Unity's Shader Graph tool, you can convert the shader code for use within STYLY. Read more about that process here: https://styly.cc/tips/shadergraph_to_shaderlab/
Lastly, you can also use Shader Forge (free) or Amplify Shader Editor (paid asset) to create custom shaders that work with STYLY.
In terms of a water shader, I find the water exaple provided by Amplify Shader Editor to be a great starting point for understanding how to create a custom water shader. You'll have to purchase and download both the Amplify Shader Editor asset (https://assetstore.unity.com/packages/tools/visual-scripting/amplify-shader-editor-68570) as well as the Amplify Shader Pack (https://assetstore.unity.com/packages/vfx/shaders/amplify-shader-pack-202484 - it's free if you've purchased Amplify Shader Editor).
Remember that you want to look at the Built-in Render Pipeline (BiRP) samples, and not the URP or HDRP samples!
I want to create a custom water shader, but I'm not sure where to start. I know that STYLY does not support the HDRP or URP, so the custom shader has to be compliant with the Built-In Renderer. Can I use Unity's Shader Graph?
Thanks in advance!