A nice sewer or sea foam effect.
A combination of model, texture, animation and shader.
Created a plane in modeling app, apply noise to the surface, flatten the plane a little so its not too high.
The shader rotates the texture across the surface of the plane and the \'noised\' surface distorts the image, rotate the plane at the same speed in the opposite direction to prevent the image rotation.
Alpha channel for transparency is written into the shader, so you can sit it on top of your water.
This might work on some folks systems better than others due to FPSC animation playback being a little laggy.
Remember to turn on shaders in preferences..
Video showing the effect of shader and animation.
http://www.youtube.com/watch?v=7TdQg1eY5DE