What shocking thing does Kit do in The Witch of Blackbird Pond? Why does she do it?

Kit does several shocking things throughout the novel. One particularly shocking event happens at the beginning of the story.  

Kit is on a small boat riding back from Saybrook to the ship. A husband, wife, and their little girl have joined them from the town. The child accidentally drops her precious doll into the water. She became extremely upset, but the captain will not turn the boat around as the toy floats away. Kit becomes frustrated and dives into the water, "plung[ing] headlong over the side of the boat." Everyone is shocked, having assumed that she could not swim. Nat dives in after her. Soon they realize that Kit does indeed know how to swim. She retrieves the toy, but everyone on the boat is still showing signs of shock, disapproval, and anger. No one forgets what Kit did and how outrageous they thought it was.