Water seeping through where pipe exits slab foundation
I prefer all slab penetrations to be sealed, especially if you are in an area where radon is such of concern to require a vent. Your local building code should spell out what is a locally acceptable material to seal the penetration but any sealant with draft stopping capabilities and made for wet areas should do the trick.

As to the water under the slab, nearly all of us have water under our slab, just some deeper than others but enough water for intrusion through the slab, crack, or otherwise in to your home is very concerning. The fix depends on a novel of conditions that could be causing or allowing this to occur.