“Go to the place where there is scarcity of water, sit and watch the clouds rise.”

 This proverb/idiom is pronounced in Chinese “Xing Dao Shui Qiong Chu, Zuo Kan Yun Qi Shi”

The intended meaning is an optimistic one. When one has reached the end of the road and faces a dead end, do not give up hope. There is always a silver lining or better days ahead and better things to come.  The analogy in the saying is that when one goes to a place where there is very little water, do not fret because we can anticipate the rising of the clouds, which is another source of water. 

source : http://proverbialchinese.blogspot.kr

So his letter is implying that even if WangSo marry WangMu’s  daugther, WangSo haven’t changed his mind. His mind will be with HaeSo forever.