I was not introduced to the world of skiing until my early 20s when I married a skiing enthusiast. Being quite poor at the time, I had no money for professional instruction, and I relied heavily on the mentoring skills of the skiing friends I had in those days. This included my wife, who had been given professional instruction in Austria.
This went on for years, and my competence slowly grew.
But eventually I sought professional instruction, where I learned that about 80% of what I had been told was dead wrong, and that the Austrian technique my wife had learned was completely out the window. I went on like that for a few more years, taking an occasional professional lesson that always tried to remove the very bad habits that had become thoroughly ingrained in my style. It became a real problem with me when I took up racing later, for when things got hairy in the gates, I kept reverting to those old habits that I would not have had if I had been properly instructed in the first place.
When I took up diving, I got a lot of advice from more seasoned veteran divers I met while diving. With my skiing experience behind me, I took it all with a grain of salt, and I now know that a whole lot of it was as wrong as the skiing tips I had received from well-meaning friends.
Last year I approached a bar at a resort in Belize to make an order, and a diver was pontificating in great detail about some of the fine points of diving to the non-diving crowd at the bar, apparently in response to a question. I lingered and listened for a while--pure, unadulterated BS. The guy left, looking very proud of himself for having passed on his knowledge. One of the patrons looked at me and said, "From the look of agony on your face while he was talking, I gather that what he was saying was not true." (The group included a doctor who knew nothing about diving but could tell that the guy's explanation of DCS was ridiculous.) I took a few moments to give them an accurate response to the original question.
I will accept the fact that many experienced divers have better skills than many instructors. The same is true in skiing. On the other hand, most experienced divers do not have the skills or training in instruction that professionals do. The problem is that it is not easy for the beginning diver to know whether the friend who seems so knowledgeable is giving you the expert advice you need, or if he is the moron I saw pontificating at the bar.