Regulatory phase-outs of legacy perfluoroalkyl acids (PFAAs) have not always led to proportional declines in marine environments, raising the possibility that overlooked precursor reservoirs contribute to persistent terminal PFAAs. Yet in situ precursor transformation in natural seawater remains difficult to verify because transport, photochemistry, matrix effects, and microbial processes are tightly coupled. Here, we conducted 6-day shipboard incubations with natural seawater collected along a nearshore-offshore transect in the northern South China Sea to evaluate the transformation potential of representative PFAS precursors. Precursor loss and terminal PFAA formation were spatially heterogeneous. In nearshore surface water, FOSAA produced PFOS yields of 15-19%, while 7:2 sFTOH produced PFOA yields of ~27%; offshore and deep-water treatments showed lower or more condition-dependent transformation. Light acted as a conditional enhancer, accelerating FOSAA decay, promoting PFOA formation from fluorotelomer precursors in selected offshore treatments, and facilitating earlier product accumulation. Transformation was also matrix dependent: N-MeFOSE rapidly formed N-MeFOSAA in natural seawater but not in ultrapure water. Compared with no-precursor controls, precursor-amended incubations showed taxonomic and predicted functional deviations, particularly in nearshore surface waters, indicating community responses to precursor-amended conditions rather than identifying PFAS-degrading taxa. These findings support the need to incorporate precursor reservoirs and spatially variable secondary PFAA formation into marine PFAS assessment.