Due to the risk of oxygen toxicity, a diver can only safely scuba dive to a very shallow depth when breathing 100% oxygen, typically around 13 feet (3.9 meters), as exceeding this depth can lead to seizures from central nervous system oxygen toxicity.
That's a much more conservative standard than I was taught. I was taught 20 ft. And my dive computer is set to 20 ft. If I went down to 20.5 feet on pure O2, it balked at me until I got back up to 20 ft, wherein it gave me the "smiley face."