Are the local residents of Beihai descended from Cantonese?

There is no such thing as offspring. Beihai people are composed of three ethnic groups: Beihai aborigines, who speak Cantonese and belong to Guangfu culture.

There are also Hakkas, mainly distributed in Hepufu, Baisha, Zhakou and Weizhou Island. Most of their ancestors immigrated from Fujian, but they still retain Hakka dialect.

There are also some Yi people, that is, the Jing people among the 56 ethnic groups. Because of assimilation, most Jing people speak vernacular. If you are interested, you can go and see the single stringed piano (unique to the Jing nationality) there.