I suspect that if a subterranea home were built so poorly that radon gas is infiltrating the walls then so would water. Both would have to be factored into the design. Modern bunkers are designed to keep radon gas out and are equipped with high flow ventilation systems. High pressure shotcrete or some similar material should more than suffice to keep gasses and water out.

Radon is a problem in traditional brick and mortar basements as those walls are typically just one layer of brick and mortar. Water and radon can easily penetrate through micro-cracks that develop over the years in traditional basements.

"I suspect that if a subterranea home were built so poorly that radon gas is infiltrating the walls then so would water."

I'm no expert, but your statement makes me ask... Why then do we have homes with basements that have radon problems, but not water problems? I don't think the two problems are always related.

Water infiltration depends on the amount of water and type of soil and the drainage designed into the basement. e.g. layers of concrete and rows of shale/rocks for drainage. Not all basements are equal. The amount of radon depends on the quantity of uranium and thorium in the soil/dirt. So you are right they are not always related but the mitigating controls can be potentially related. Some construction companies just focus on maximizing profits in my opinion and some individual home builders will use construction dirt that contains low or no uranium or thorium.