in America about 36% percent of people can't swim and world wide about 54% can't swim.

2nd answer:

People don't tell them that. While a small percentage will still not like the water, most people have to be told how to float before they how to then use their limbs.

People try to mimic others by dog paddling not knowing how to stay afloat any other way. It doesn't work.

So, tell your friends that don't know how to swim, "keep your lungs at least half full to stay buoyant".