You say, “… I believe you can do a 150m dive or a 200m dive and come up safe without a lot of training. But then it is just a good luck…” I don’t believe that statement. To state that in a forum is to give some people an idea that maybe they can do it too. This type of dive is one which should simply not be attempted. If divers need to be at that depth, there are other means to do so (saturation diving with mixed gas). You cannot depend upon how a diver “feels” to do this kind of dive, as some conditions will come without any advanced warning.
SeaRat