Code may require that you fill in the 'access pit' because of radon gas.
As far as ground water seepage, the concrete won't prevent a rising water table. The plastic sheet under the foundation (assuming you have one) is to inhibit water vapor. But rising water will come around and through the barrier and up thru the floor, regardless of what's there. If you ever have a water problem, you have to fix it with a sump or improving your land's drainage.
I know some places admit the use of a studor or other type of air admittance valve for bar sinks or islands - but I am not sure they're permissible for tubs and toilets. Check w/bldg offc, since this'll be specific to yr municipality.