It made perfect sense for a 19th century boat to be docked in 18th century Liberty Square?
The Hudson had sidewheeling boats. The boat in RoA was a rear-wheel Mississippi boat. Twain makes mention of loving "this river," which is obviously meant to be the Mississippi. But at no point do they make mention of being on any other river, American or otherwise. Twain never commanded a Hudson River steamboat.
And again, to all: All of Liberty Square through Frontierland is hand-wavy mix of settled (not the frontier) and unsettled (the frontier) places full of geographical inaccuracies ("The South" = "The Frontier"!!) and anachronisms. The bears that could sing 1950 songs should have been a clue that you ain't gonna find a neat and tidy theme or any sense of historicity since it includes fantasy.