"The sea is a world of dragons, and the clouds are the hometown of cranes" is actually a couplet. The correct one is "The sea is a world of dragons, and the clouds are the hometown of cranes." It was a couplet given to Chairman Mao Zedong by Mr. Qi Baishi own work.
The meaning of this couplet is that dragons grow in the sea, and the sea is where dragons move. Cranes can fly high and soar in the sky, so the clouds are like their home. This is actually a metaphor, indicating that the dragon was born in the magnificent sea and has great strength and ability, while the crane's home in the clouds is a symbol of nobility.
Extended information
The anecdote of "The sea is the world of dragons, and the clouds are the hometown of cranes"
After Qi Baishi sent Mao Zedong's paintings, the famous collectors Zhang Boju and Wang Yue and others came to visit, and Qi Baishi happily talked about sending two works to Chairman Mao. When it came to the seal script couplet "The sea is the world of dragons, and the clouds are the hometown of cranes", Zhang Boju couldn't help but let out an "ah". It turned out that there was a wrong word in this couplet.
The latter couplet, written by Deng Shiru, a native of Wanbaishan, Anhui Province in the Qing Dynasty, was originally called "Heaven is the hometown of cranes", but Qi Baishi wrote "cloud". Qi Baishi immediately became nervous after being reminded by Zhang Boju. Chairman Mao was a well-read scholar who was well versed in ancient and modern times. To send him a couplet with typos was not only disrespectful to Chairman Mao, but would also make people laugh if it were spread out!
Zhang Boju hurriedly comforted the old man and said: "Mr. Qi, your change of the word 'Yun' is better than Deng Shiru's word 'Heaven'. If the first line is 'Earth', then the second line 'Heaven' cannot be changed." But the first couplet is the word "海", which is exactly opposite to your word "cloud". We don't have to stick to the formal pattern. Changing the ancient sentences has been around since ancient times. Chairman Mao may praise you for your good change!" Zhang Boju said this! As soon as he said this, Qi Baishi calmed down.