This seems to be a pretty hot topic on ScubaBoard these days. I am sure you must know that a dive shop owner in Cozumel died recently after just such a dive. The plan was to go to 300 feet on air and then surface. I am sure that if she were alive she would tell you that the narcosis at 300 feet felt pretty much the same to her as it did at 135 feet. That's one of the problems with narcosis--you usually don't feel it enough to realize what it is doing to you. In her case, she apparently did not realize that she had reached the 300 foot depth and kept going. A buddy caught up with her at 400 feet and turned her around. That extra descent used up their air supply, so they ran out of air at about 200 feet. Fortunately, they had another diver with them who had not gone past 300 feet and who still had enough air to get them to the surface (no stops) with three person buddy breathing. She died. The buddy who caught her at 400 feet is still paralyzed. The other diver will probably recover, although we are not getting reports in him so I can't be sure.
Let's start with narcosis. It affects different people differently, and it affects the same person differently on different dives. Frequently it is not a problem on dives like that, especially if nothing out of the ordinary happens. If something out of the ordinary happens, you may not have the mental agility to deal with it effectively. You may also be numb to circumstances around you, missing vital clues, like the reading of your depth gauge. So it's a roll of the dice. Usually it won't be a problem. Sometimes it will.
You don't mention how much air you brought with you. You evidently had enough to reach your depth and reach the surface. Great! Did you have enough to deal with an unforeseen problem, like a free flow, a blown o-ring, or a buddy who unexpectedly descends an extra 100 feet and needs to be caught and returned to the surface? So taking just enough air to get you down and back is a roll of the dice. Usually it won't be a problem. Sometimes it will.
You did not mention any decompression planning. Did you use any established tables or computer algorithms, or did you take a good guess? Decompression sickness is hard to predict. Diving within recreational limits is extremely safe, and the incidence of DCS on such dives in extremely low. As you go deeper, the need to follow decompression schedules becomes more and more important, and the odds of having a DCS incident such as the one I described above increase. If you are following a computer, do you have a backup in case it fails? If you are following a plan or a computer and and are able to do the stops because nothing unforeseen happened and you have enough air, you are likely to be fine. So it's a roll of the dice. Usually it won't be a problem. Sometimes it will.
I am not sure how much experience you have dealing with equipment issues at depth. If nothing goes wrong, you will be OK. If something does go wrong with the equipment, do you have the ability to deal with it at depth? If not, it's a roll of the dice. Usually it won't be a problem. Sometimes it will.
Technical dive training results in divers using gas mixtures that minimize (but don't eliminate) the effects of narcosis. These divers calculate their gas needs carefully and carry enough in case of emergencies. They train for equipment emergencies. They have carefully planned decompression profiles and backup plans in case of emergency. They have well established protocols for dealing with every possible situation, and they know how to work effectively in buddy teams in case of emergency. In short, they have thought of everything that can go wrong and made plans to deal with those rare but anticipated problems.
So, yes, there are people who do this kind of diving regularly. They seem to enjoy that roll of the dice. Usually they don't have a problem. Sometimes they do.
