Unfortunately, diving at depth doesn't allow for many errors. I think the original Cousteau expedition dived on air? These days you use a mixed gas set up (and often a rebreather, which is much less harmful to surrounding hull structure on penetration dives than open-circuit scuba), but it still involves limited bottom times and in-water decompression.
Just to clarify for the non-divers, Nitrogen Narcosis is not the same as Decompression Illness (DCI). Narcosis is caused by breathing elevated gasses at pressure. The effect is similar to intoxication - I've heard it called "the martini effect", as after it kicks in it feels like you've slammed down a martini for every 10 metres or so you descend - although that's not a hard and fast rule, as it effects different divers at different depths. One of its other names is "raptures of the deep", because it usually gives you a feeling of well being (although some people can hallucinate in extreme cases). It acts like Nitrous Oxide - laughing gass. Problem is, it impairs judgement and can give you inflated confidence and a sense of well-being. I've had it kick in on open-circuit air big time at around 40 metres - the world was just absolutely fabulous, and it was all good. Fortunately, most divers recognise the symptoms and respond - it can easily be counteracted by moving to a shallower depth. Unlike alcohol, its effects come on suddenly and can be almost immediately alleviated by going shallower. One of the things tested for in advanced recreational diving courses is the impact of nitrogen narcosis - divers are given a slate at depth and asked to do simple sums and see how long it takes them. I've heard some hair raising stories about what people can get up to when they're "narked" and of course it can be a major contributor to accidents as it hinders judgement.
DCI is far, far more serious (I've been narked, but I've never had DCI - and hope never to experience it). Simplistically explained, it's a form of baramtrauma, or pressure injury, and is caused by the expansion of gasses that accumulate in body tissues at depth. As we ascend, these gasses expand, form bubbles and cause damage to the tissues. There is a range of severity, from mild to extreme. People, again, react differently, and the exact perameters for getting a "hit" are still being studied.
This is why divers at depth have limited bottom times according to how deep they go (the longer and deeper you are, the more nitrogen is accumulating in your tissues), and have to stage their ascent to the surface to off-gas slowly. It is also why a diver suffering from DCI will be put in a decompression chamber that will return them to the pressure at which they absorbed the nitrogen, and then gradually bring them back to the surface atmospheric pressure, or some commercial and other divers who can't spend time in-water decompressing will use a chamber.
The "bends" is one manifestation of DCI - it is when the bubbles formed on ascent are near joints. The term is used more widely, though - even divers who did not experience this joint pain but who had other forms of DCI are often said to have been "bent" in colloquial dive-speak.
Divers keep a close eye out for symptoms - one warning sign is a tingling or cracking under the skin, an indication of microbubbles forming. Left untreated, there is increasing chance of injury or even death. It takes some time even after surfacing to completely off-gas - multiple dives on successive days will increasingly limit your depth and the amount of bottom time you have, and divers wait at least 24 hours after their last dive before flying, as the sudden change of pressure can bring on DCI. You have to be on watch for potential DCI symptoms for a couple of days after a dive.
