Know your own limits. In the old days with minimal (or no) certification people used to dive in caves and take off tanks to squeeze through small gaps. I am sure a few people still do.
But as training has become more formalised, and people more litigious, the official advice has always been: train properly and equip properly for more hazardous aspects of the sport. It's good advice.
But I don't have a problem with even relatively inexperienced divers going into "overhead" environments provided conditions are benign and however far in you go you stay safe and within your own limits. When you are directly underneath your boat or poking your head under a ledge you are in an "overhead" environment, so it is somewhat a matter of degree. The mouth of a cavern or just inside is not the same as some deep penetration under ice or into a cave or wreck.
But as training has become more formalised, and people more litigious, the official advice has always been: train properly and equip properly for more hazardous aspects of the sport. It's good advice.
But I don't have a problem with even relatively inexperienced divers going into "overhead" environments provided conditions are benign and however far in you go you stay safe and within your own limits. When you are directly underneath your boat or poking your head under a ledge you are in an "overhead" environment, so it is somewhat a matter of degree. The mouth of a cavern or just inside is not the same as some deep penetration under ice or into a cave or wreck.