The oar being farther away than she thought, and rapidly receding, caused her to lean far over the boat, and in his effort to get his oar again in position, King, too, leaned over the side.
The result was exactly what might have been expected.

The narrow, clinker-built boat capsized, and in a moment the four children were struggling in the water.
Even as the boat went over, King realized what had happened, and realized, too, that he was responsible for the safety of the three girls.
With fine presence of mind he threw his arm over the keel of the upturned boat and shouted, "It's all right, girls! Just hang on to the boat this way, and you won't go down." Marjorie and Molly understood at once, and did exactly as King told them.
They were terribly frightened, and were almost strangled, but they realized the emergency, and struggled to get their arms up over the boat in the manner King showed them.
But Kitty did not so quickly respond to orders.

She had not been paying any attention to the merry war going on in the stern of the boat, and when she was suddenly thrown out into the water, she could not at first collect her scattered senses.

King's words seemed to convey no meaning to her, and to his horror, the boy saw his sister sink down under the water.
"Hang on like fury, you two girls!" he shouted to Marjorie and Molly, and then he made a dive for Kitty.
King was a good swimmer, but, hampered by his clothing, and frightened terribly by Kitty's disappearance, he could not do himself justice.

But he caught hold of Kitty's dress, and by good fortune both rose to the surface.