It is not that water can block ultraviolet rays, but that water can absorb some ultraviolet rays and reflect a considerable part, giving primitive life a good barrier.

After Proterozoic, photosynthetic autotrophs, led by cyanobacteria, appeared, and the oxygen content in the atmosphere rose sharply, forming an ozone layer over the earth, shielding the threat of ultraviolet rays to life and making landing possible.