Is it normal to think if your not olympic swimmer you cant be marine/coasty?

Why do people believe if your not the greatest swimmer in the world you cant be a Marine/Coasty(Coast guard)? I heard of people who could not swim when they went in graduating from Marine boot camp. Its not rocket science. You can teach a person to swim. No I not saying they are the same thing either. Just people seem to assume you cant enter either branch unless you are athletic swimmer ability. Which is complete BS.