Why is the sea water in Fangchenggang so dirty?

Management error. According to the website of Guangxi Fangchenggang Ecological Environment Bureau.

1, the urban sewage treatment plant failed to play its role effectively.

2. The remediation of sewage outlets into the sea (river) is not thorough enough.

3. Fangchenggang is a prefecture-level city under the jurisdiction of Guangxi Zhuang Autonomous Region, an important node city of urban agglomeration around Beibu Gulf, a coastal city, a border city and a port city.