Actually, I accidentally took and passed the South Dakota course. I moved the mouse without noticing. I was almost through the test when a question came up about South Dakota registration (which isn't about safety anyway).
I never got around to taking the SC test. In SC, you don't have to know how to operate a boat safely (or at all) to operate one.
There's a marina near me that advertises "Boat rental - no experience necessary."
The "Boat rental - no experience necessary" sounds rather interesting... but in regards to the initial question you can always find out state rules and regs by going to www.boat-ed.com and selecting your state. Specific to DC, we actually have a course for DC (http://www.boat-ed.com/districtofcolumbia/boating_law.html). And, it's good throughout the states.