The addon is in the N panel, the preset scenes are in the "preset scene" blend file that can be downloaded besides the addon. The preset scenes are the best way to start off your projects - just use the save as function of Blender to create a new file based on a preset scene or append a scene from it. Alternatively you can append a scene from the save file.

Occasionally, the addon might not work/appear after installing it for some people. What to do: 1. Close Blender and re-open it again. This most of the time fixes it. 2. Delete the addon and reinstall it.

The sharpness and strength of caustics depend on the size and strength of your light source. For example the default Nishita sky creates extremely bright caustics by default, so you need to make the whole sky and the sun way less bright. Ultimate Sky works correctly out of the box with the caustics and the preset scenes are already set up to look the best with the water materials (even with Nishita Sky). Sun lamps and other light sources (HDRI, other Sky addons) might also work differently so you may also need to change the sun brightness.

Use the "caustics sharpness" slider on the materials/shader UI to change how strong/bright the caustics are, usually best look around 0.6-0.7 values - also Caustic type 1 is brighter by default than caustic type 2 so it also depends on which one you decide to use in the shader.

Smaller shaders are good if your GPU struggles to display the complex water material in the viewport shading mode (in Eevee). In Cycles, there is no difference between the two type of shaders that is included. Since viewport shading is using Eevee instead of Cycles, the slow compiling time on the Main shader makes it a plausible reason to use the smaller shaders even when you are creating a scene for Eevee.

UWS3D v3.1-4.2 support Blender 4.0 and up. V2.6 to 3.0 work between Blender 3.0-3.6. Older versions work in Blender 2.91 to Blender 3.0.