How do you seal a cast iron pipe coming through concrete when the pipe
penetration is below the waterline???

I am going to pour concrete around the shower drain pipe in the
concrete slab and create a drywell. However, the rusty cast iron pipe
will go through the concrete slab below the water line (during the
rainy season). I need to seal the seam between the cast iron pipe and
concrete so no water will infiltrate into the drywell I am creating. I
have no basement; this is on the ground floor of my home.

Any and all valuable advice is welcome and would be greatly appreciated!