Hi Janelle_5719,
In class I also mentioned that specific gravity can be taken relative to anything, not necessarily water. The formula in the CC specifies water only because that is most commonly how specific gravity is determined.
Also, specific gravity is just a relative density concept. So that means the densities in the equation do not have to be liquid. You can take the specific gravity of anything (solid, liquid, or gas) relative to anything else.
For your last questions, yes, % apparent weight happens to equal 1/specific gravity.