I'd never heard of the hundred-foot rule, so I googled it and found a quotation:

"It's called the 100 foot rule. If you're within 100 feet of another object -- another watercraft, a dock, a swimmer -- you're supposed to slow down to idle speed. If people did that almost all these accidents could be avoided." -Lt.

The source is the Forsyth County Sheriff's Office, wherever that may be.