Short explanation and then it's up to you: make a terrain (no need for far mountains), with low thick soft cloud (even without PF), maximized at water level you intend by distribution shader, and maybe a distance shader for additional control. I colored it quite dark. For god rays, make another cloud a bit higher up above water level, and experiment whether it does any good. I used small fractals and quite strong contrast. The sun needs to be in front of you for star-like rays.
The water is a sphere, same size as the planet, but with a minus before radius, no shadows, and as a surface I use a surface shader without color. In that, you raise the water level by the displacement offset. And then add you water shader as a child to this surface shader. Thats basically it.
