The complete poem "Farewell to Yishui" by Luo Bingwang, a great poet of the Tang Dynasty, is: Farewell to Yandan here, the strong man will be proud of his crown.
The people were gone in those days, but the water is still cold today.
The "Yishui" in this poem is in today's Yixian County in northern Hebei Province. During the Warring States Period, the knight Jing Ke was entrusted by Prince Dan of Yan State to go to Qin State to assassinate the King of Qin and win the government. Before leaving, Prince Dan of Yan State and others saw him off in Yishui.
So, "Yan Dan" in this poem refers to: Prince Dan of Yan Kingdom. "Strong man" refers to: Jing Ke.
I hope my answer can help you.