Certification is not necessary to dive. A certification card is merely a means to show that a certain level of training has been satisfactorily completed. Training is really the key to keeping safe.
An untrained diver can absolutely do a dive in shallow water and live to tell about it. Their odds on a successful outcome increase with training. The training is more about teaching the diver some rules of physics that can’t be ignored (don’t hold your breath) and also giving the diver some skills to cope with some problems that may arise (mask/regulator recovery, air sharing, etc.)
How much training is needed will depend on the diver and the dive. It’s not one size fits all. Personally, for me, I’m not taking anyone diving off my boat that isn’t certified. Too much can go wrong, leaving me open to potential legal issues. Plus, I’m not wanting to babysit an untrained diver. I will, and have, taken newer divers out for easier dives to ease them into it, but they’ve all been trained and had the background knowledge so I didn’t have to check them every second.