<p>Vernacular marine knowledge that is originally created by, and circulated among, the indigenous groups is usually appropriated and adapted by authorities and markets to achieve various geopolitical, economic, and social ends. This phenomenon is particularly pronounced in the territorially contested South China Sea (SCS). Folk and indigenous heritage, customs, rituals, and knowledge in relation to the maritime space are utilized and proclaimed as historical evidence to boost maritime sovereignty claims. This research investigates and interrogates the geopolitical relevance and resonance of the continuing evolution of the Chinese marine knowledge record Genglubu, a living part of seacraft and guides written by the Chinese fishermen in Hainan Island recording navigation routes, meteorological, hydrological, and other information for fishing in the SCS. Drawing from assemblage approach, we see Genglubu as an assemblage of marine knowledge (re)production which is engaged and shaped by multiple actors possessing diverse desires and constituted by complex, intertwined processes of human-sea-land interactions. The emergence of Genglubu originates from Hainan fishermen’s daily encounters and use of the SCS, while the transformation and evolution of Genglubu are intimately embroiled in the fierce, dynamic territorial rivalry in the SCS. We also find that the changes to Genglubu risk may alienate local fishermen. By focusing on the relationship between vernacular marine knowledge and the territorial geopolitics playing out across multiple scales, this research advances understanding of the complex geopolitical entanglements surrounding maritime knowledge.</p>