Thats true dv8, my father insisted at our locacl community swimming pool that I was to be learned how to swim properly at the age of 4 despite the fact that officially they only teach as from the age of 6. It took not much convincing the head swimming master, although some dispute, so my father just let me go and the first thing I did was just jump into the deep end of the pool.
After doing that a couple of times to convince the teacher that it was no coincidence, I was taught how to swim properly at the age of 4. Both my parents are also very good swimmers.
But still then after learning how to swim properly, my father kept a very close eye on me and so did the pool master because kids will be kids and sometimes my own fault but often the fault of other bigger children, I went under a few times.
Other example how stupid and quick something like that goes : stephdaughter, 5 years old, cannot swim yet, has already a bit basic, insists on going on the steps in the water at a waterpark where the big tube comes out to see better how people are yelling when they fly out of that tube. Since she was standing on the steps and not swimming, she had not protection on her arms. She takes one step further and blup blup...there she went under and stayed under...her father jumped in immediately with his clothes already on...stupid of us? Sure but who would have ever thought that she would take a step further...hmm