Is it possible to use layers with fog?

So i'm trying to create an effect with water so that it will have fog underneath. I know with unity pro you can have volumetric fog, but i'm using the free version. So i did a little thinking and thought about layers. I'd seen the boat tutorial, where the boat is semi-submerged in the water plain, and has another plain modeled on the inside with a material that makes the water plain disappear when you look through it. So the effect is that inside the boat it looks like there's no water.

What i'm looking for is the ability to do the opposite with fog. Create a plain, that when you look through makes the fog visible, instead of it being visible everywhere except through the plain like with the boat. Does anyone know how to do this? or if it's even possible?

As always, any help or advice is greatly appreciated. Thank you for your time.