It only works correctly in Blender 2.81 and later versions.

Use the UV map on your plane and rotate it.

The wave size is adjusted to an aproximate realistic size. The Ultimate Water Shader has "bigger" waves relative to the plane you use it on when you use the clean and dirty water, as they should be used on small planes. If you use them on big planes, the waves won't be that visible. Resize the plane and apply scale to see details better.

The shader fakes depth and translucency which basically looks like volumetric shading so there is no need to add volumetric nodes under the water plane or to the Ultimate Water Shader.

If you want to accompany the shader with volumetric object underneath anyway, make sure to set the transparency to maximum on the Ultimate Water Shader (this method was never tested by me, might look unnatural).

Always use a plane, as the material is optimized to be used on a plane. Using it on a 3D object will produce an unnatural look.

Partly. Eevee produces artifacts with the Main shader and has slow compiling times. The included smaller shader nodes in the 2nd material are much better when using Eevee and they can be mixed too with a mix node and mixrgb node manually. Eevee version by default doesn't have transparency.

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.

Everything is pre-animated, no need to animate it yourself.

To change the animation speed use the 2 default keyframes (pre-animated 5000 frames). Select the Ultimate Water Shader and tab into the shader group. Then select a sub-shader and the corresponding keyframes will appear. Move these to change speed. Flip the animation direction by flipping the keyframes.