The seed values are not continuous or adjacent so varying the seed will not create a smoth variation. You will have to animate other parameters, for example scale and position. The Transform shader is useful for this, as OB mentioned. It allows you to "transform" the noise function shader that provides the shape of clouds and water with the "translate" function. Translating in the x and z directions will move the shapes along the water or cloud plane (maintaining the same shapes and moving them horizontally across the surface) while translating along y will actually vary the shape of the texture in a smooth way. Translating along y moves the volumetric fractal shape through the plane of the water or cloud node vertically. Only a small portion of the virtually infinite fractal form is actually visualized by the cloud or water shaders, so by moving the procedural texture "through" the node's visualization plane you get smooth variation.
- Oshyan