Complex sentence: Lincoln's law partner and biographer, William H. Herndon, also reports an additional incident at the time: a boat Lincoln was on got stranded on a shoal; the boat gradually swung clear and was dislodged after much manual exertion.
Split into simpler sentences: Lincoln's law partner and biographer, William H. Herndon, also reports an additional incident at the time. The boat gradually swung clear and was dislodged after much manual exertion.