"Water can carry a boat, but it can also overturn it" comes from Xun Zi Ai Gong, which is a dialogue between Confucius and Lu Aigong. It means that rulers are like ships and people are like water. Water can not only make a ship sail safely, but also capsize and engulf it and make it sink into the water. It is roughly equivalent to "those who win the hearts of the people will win the world, and those who lose the hearts of the people will lose the world", which also means that things are used in a favorable way and are used in a unfavorable way.