Obviously you got the idea from my snorkeling thread ... and you're right... it is assumed most people know how to swim... depends on your upbringing, depends on the people you hang out with, depends on whether or not your school required it.
Well I never learned during my childhood because neither of my parents knew how and I never needed to... my mom is deathly afraid of the water. In HS, the phys ed class required swimming and some of us got excused for it (my excuse was allergies to chlorine). It wasn't like they taught you either... they just made you do it and I suppose they ASSUMED everyone already knew how.
So by college, I signed up for a non-swimmers class and the grade was based simply on attendance. It takes me a lot of courage to swim the length of the pool with the proper method of breathing. I think I was successful only a couple of times. I haven't had much practice since then and so I've lost all technique... so I consider myself a non-swimmer. I can do the dead-man's float on my back... that's about it. I think I just need to work up the courage to do the breathing correctly without taking water up my nose or choking on that water I drink while inhaling... that blows... and it certainly doesn't give me incentive to try it again on the same day...