Do you have to swim in Boy Scouts?

Not necessarily. There is a swimming merit badge but it is not required. One of the requirements for Eagle Rank is to earn either the Swimming, Hiking, or Cycling Merit Badges. If a Scout can't or doesn't want to swim, he can opt for one of the others.

Also keep in mind that if a Scout cannot complete any of those merit badges due to a disability, he may apply for an alternate requirement. This is true for any of the Eagle Requirements.