Someone will jump in with the official name of the phenomenon I am about to describe, which essentially is that people who are not themselves highly competent in an activity are not able to judge high levels of competence and will frequently mistake mediocrity for a high skill level. I think a lot of that is at play with the people who scoff at the need for advanced instruction and say "just go out and dive."
If someone learns to dive in a typical resort environment and then spends 20 years diving in a variety of typical resort environments, that person will become accustomed to the various levels of diving typically found there. That person may accurately perceive himself or herself to be among the best of that lot of divers. Speaking for myself, I was an instructor with several hundred dives before I did any serious diving outside of such an environment, and I thought I was pretty good. In that context, I was pretty darn good. It would be very easy for such a diver to think, "Hey, I got to the top by just diving, so anyone else can, too."
But then that diver may decide to go a different route--say real technical diving, cave diving, etc. While laying line in a high silt environment, the tanks scraping the cave ceiling and the stomach an inch or two from the silt below, that diver may realize that the skills required to do that without kicking up a silt whirlwind are different from those required to swim with the body at a 45° angle while 10 feet away from a coral reef. My entry into tech diving gave me my first glimpse of the kind of skills that requires, and I was thoroughly humiliated.
So, yes, if you aspire to be the kind of diver who only dives at vacations at tropical resorts, then you can be among the best with only experience as your guide. If you want to go beyond that, you are going to need some guidance.
If someone learns to dive in a typical resort environment and then spends 20 years diving in a variety of typical resort environments, that person will become accustomed to the various levels of diving typically found there. That person may accurately perceive himself or herself to be among the best of that lot of divers. Speaking for myself, I was an instructor with several hundred dives before I did any serious diving outside of such an environment, and I thought I was pretty good. In that context, I was pretty darn good. It would be very easy for such a diver to think, "Hey, I got to the top by just diving, so anyone else can, too."
But then that diver may decide to go a different route--say real technical diving, cave diving, etc. While laying line in a high silt environment, the tanks scraping the cave ceiling and the stomach an inch or two from the silt below, that diver may realize that the skills required to do that without kicking up a silt whirlwind are different from those required to swim with the body at a 45° angle while 10 feet away from a coral reef. My entry into tech diving gave me my first glimpse of the kind of skills that requires, and I was thoroughly humiliated.
So, yes, if you aspire to be the kind of diver who only dives at vacations at tropical resorts, then you can be among the best with only experience as your guide. If you want to go beyond that, you are going to need some guidance.