Underwater habitats could probably be made with plastic still. Under a few hundred feet, radiation will probably drop off enough to not cause damage. Of course corrosion becomes a new problem to fight.
At a few hundred feet, a structure able to hold off the water pressure would probably be thick enough to hold off the radiation if installed on the surface. In rough numbers, the mass of air above us now is equivalent to about 10m depth of seawater. A few hundred feet, call it 100m, would be like having ten atmospheres worth of radiation absorption above us. We need not go that far. Relatively thin engineered coatings on glass/plastic sheets would be as effective at blocking the extra UV.