Can you navigate down the Congaree River from Columbia South Carolina to the Atlantic ocean by motor boat?

Yes. I have made the trip a dozen times. Water levels vary, but it is usually navigable with six feet of water under the hull. It is a beautiful trip, which passes the Congaree swamp, the "high hills of the Santee", where you can see Mountain Laurel and dogwoods bloom at the same time. It joins the Wateree River to form the Santee River. Lake Marion is the first of the Santee-Cooper lakes, connected to Lake Moultrie by a diversion canal. The Pinopolis lock allows boaters to descend 73 feet to the Cooper River. The Santee-Cooper project envisioned barge navigation from Columbia to Charleston. There are actually a couple of abandoned wooden barges on the Congaree River. It is a beautiful trip that I highly reccommend,