Plasmonic nanostructures have shown great promise in solar thermal applications. Here we report highly efficient solar vapor generation using hollow-mesoporous plasmonic nanoshells, where the steam production is significantly enhanced by the extended active surface and the nearly neutral buoyancy. An energy-to-vapor conversion efficiency of 69% was measured under 10x solar irradiance. We also show clear evidence of direct vapor nucleation from the plasmonic structure without bulk heating.