Blue-economy growth is promoted as a pathway to sustainable coastal development, but its climate implications depend on how marine demand reshapes emissions across land-sea supply chains. Here we construct city-level environmentally extended maritime input-output tables for China’s Bohai, Hu-Hang-Yong, and Guangdong-Hong Kong-Macao Bay Areas to compare marine and non-marine carbon efficiency across direct, indirect, total, and upstream-city effects. Marine sectors generate lower emissions per unit of value added than non-marine sectors on a complete supply-chain basis, and this advantage extends to inland suppliers through national production networks. Across 81-96% of upstream cities, marine-induced value-added growth is decoupled from associated emissions, while 56-62% exhibit stronger carbon efficiency than comparable non-marine demand. Regional attribution shows distinct mechanisms with upstream production depth in Bohai Bay Area, network embedding in Guangdong-Hong Kong-Macao Bay Area, and mixed channels in Hu-Hang-Yong Bay Area. These results position marine supply chains as anchors for climate-compatible blue-economy governance.