Oxygen becomes toxic at certain pressures. On 100% oxygen your safe depth is only about 40 ft to avoid oxygen toxicity. As you said it depends on the person.
And it even varies for the same diver at different times and under differnt conditions. 30 and less was considered “safe,” but sometimes folks got in trouble or even passed out at that “safe” depth. Others routinely took oxygen to 60’ without problems.
Usually, we were restricted by a harbor depth to about 25-30 feet, so we were literally skimming along the bottom, hoping not to run into old piles of nets or junk on the bottom.
In daytime, skimming just above the sand or mud, you would be able to see how much you were “crabbing” due to current in relation to the bottom.
We trained in daylight and at night, and of course, we had no lights, just the glowing dials of the watch, depth meter, and compass attached to our plexiglass “attack boards.” One buddy with the attack board “navigating” by time and course, the other just touching his shoulder like a “wing man.”