Drowning doesn't look like drowning. The thing they show in movies or on TV is not what happens so most people have no clue when someone is in trouble. In some cases kids drown in pools while parents are not only present but looking right at them. In others like yours people wave and smile from the beach as the person in the surf is about to die.
Mayo Clinic: https://www.youtube.com/watch?v=h0FH9b1LiwY
WebMD Spot the Drowning Person: https://www.youtube.com/watch?v=beNheoRRdKk
Another one is Bondi Beach - you can find episodes on YouTube. There are a bunch of examples of rescues and how little notice any of the people on the beach or other swimmers nearby give to people struggling.