Hi,
I wanted to add some water (standard EE Lake) to my scene, but it seems like it isn't possible for mobile.
When I deployed the sample scene to android all I saw was a blue plane...

Is there a way to disable realtime reflection for the water and enable some simple cubemap reflection instead?

added to the roadmap "improve water for mobile"

Okay, thanks.

Is it possible to write custom shaders for mobile games? So could I just write a simple shader with animated textures and cubemap reflection by myself and apply it to a plane?

Mobile's use OpenGL, and currently creating opengl shaders is available only on Mac.
If you have Mac then you could, but you'd need to use custom Mesh instead of Water, because Water does not support setting custom shaders.