I don't know how many of them there are, but I imagine they're named for the main characters in the book. Jim's pretty minor in it, and I doubt the Judge, Joe, Sid, Muff or Mary would make the cut.
So some combination of Tom Sawyer, Huckleberry Finn, Becky Thatcher, Aunt Polly, Injun Joe, and Widow Douglas would be my guess. Since I don't know how many rafts there are, I'll go with all six as my answer!