when I was small, (not a toddler) my folks went to the lake an we had an inter-tube, (guessing about 4 years of age), I was floating in it near the shore, but go the idea of paddling it out I was out a few hundred yards before they saw me,
that was many years ago, even if your paying attention things can happened.
once the kid had gone beyond what was safe for an average swimmer could safely swim it was up to a boat, and most likely by the time a boat was called in the kid could have floated out much further, (who knows possibly some family member was on the boat taking the pictures)
For example:
I know when I was about 19 I got the dumb idea of swimming across a small lake may be 100 yards, and was getting very concerned about making it to the other side, and thought I was a good swimmer, not as good as I thought,