We were told to be aware of narcosis somewhere after 30m. We were also told the maximum diving depth (56m) caused by oxygen partial pressure. And then we were certified to dive to 18m.
The complexity of the narcosis phenomenon makes explaining it accurately very time consuming. In dive business time is money.
Cold stress and light hypothermia also cause narcosis, as do fear, hunger, trouble at work, immersion, ... I do feel a slight mental impairment at least at 27-29 meters depth. A certain slowness in awareness and response and a kind of relaxation as compared to the better awareness at shallow depth (and especially warm water abroad). Dexterity is also slightly affected. At 40 m narcosis causes a noticeable mental and sensory impairment and "stiffer fingers", and everything becomes much harder to do. Adding the effects of low visibility and darkness and a general lack of visual references into this can sometimes result in 'interesting' sensations. Visual or balance. Deeper diving than that becomes very unpleasant. I have experienced, and documented, how higher logic fails at these depths. And how issues thought to be resolved, are not. How awareness of time may sometimes be hard to maintain. It is no longer enjoyable.
Explaining things like these in full detail, and then aswering questions, takes a lot of time. Hours, possibly. It is more cost efficient to say that after 30m/100ft there is narcosis. Period. It may also be better from a liability standpoint. Not everybody can be an amateur instructor.