The URL for babylon.waterMaterial.js in the documentation seems to be incorrect.
Perhaps the following URL is correct now. https://github.com/BabylonJS/Babylon.js/blob/master/legacy/dist/preview%20release/materialsLibrary/babylon.waterMaterial.js
→ https://cdn.babylonjs.com/materialsLibrary/babylon.waterMaterial.js.
I found a similar broken link in the Doument of Babylon.js.
Babylon.js also has a number of pre-built procedural textures that are gathered in the Procedural Texture library: https://github.com/BabylonJS/Babylon.js/tree/master/proceduralTexturesLibrary
I would be grateful if you could correct this as well.