We report the results of a series of molecular dynamics simulations of water inside a carbon-slit pore at supercritical conditions. A range of densities corresponding from liquid 0.66 g cm−3 to gas environments 0.08 g cm−3 at the supercritical temperature of 673 K were considered. Our findings are compared with previous studies of liquid water confined in graphene nanochannels at ambient and high temperatures, and indicate that the microscopic structure of water evolves from hydrogen bond networks characteristic of hot dense liquids to looser arrangements where the dominant units are water monomers and dimers. Water permittivity was found to be very small at low densities, with a tendency to grow with density and to reach typical values of unconfined supercritical water at 0.66 g cm−3 . In supercritical conditions, the residence time of water at interfaces is roughly similar to that of water in the central regions of the slabs, if the size of the considered region is taken into account. That time span is long enough to compute dynamical properties such as diffusion or spectral densities. Water diffusion in supercritical states is much faster at low densities, and it is produced in such a way that, at interfaces, translational diffusion is mainly produced along planes parallel to the carbon walls. Spectral frequency shifts depend on several factors, being temperature and density effects the most relevant. However, we can observe corrections due to confinement, important both at the graphene interface and in the central region of the water slab.