Long ago in The Kingdom of Jushi, the sea was simply divided into the East, South, West, and North Seas. The Yellow Sea has only been around for about seven or eight hundred years, and it is called the Yellow Sea because, well, it's yellow.
So when Li Bai bragged back then, "I once sailed above the East Sea"... The East Sea he mentioned refers to the Yellow Sea near Qi Lu.
Hence, Gu Yu brought Xiaozhai back from Putuo to the Qi Lu sea area and even made a special trip to Tiantai Mountain.
Tiantai Mountain was famously renowned as the place where the sun rose in ancient mythology, where figures like Di Jun, Xi He, Tang Gu, and others resided. Later, Heshang Gong founded the Dao here and took on An Qisheng as his disciple. Even Qin Shi Huang and Emperor Wu of Han Dynasty sought the elixir of life here and, failing to find it, erected the Wangxian Terrace on Tiantai Mountain.