Adobe Certified Expert (ACE) Liran Tabib covers how to create an underwater scene in After Effects, using only Ae standard plugins, with this ~10 minute tutorial that relies on fractal noise.
Fractal Noise in After Effects is a utility that can have a range of uses, especially in production. With it, you can generate natural looking patterns, animation, and use it to build up effects. Need to distress a layer? Need to animate special effects? Fractal Noise. The versatility of noise is unparalleled in computer graphics.
The built-in Ae effect can also be used to put together realistic scenes, as shown in Tabib’s tutorial. Fractals are the basis for creating the water caustics on the seabed floor. They are also used to create the waves on the water’s surface. Basic loopOut expressions in Ae will save some keyframes when animating some of the effects.