This returns the density of saturated liquid 958.367 kg/m3:
ThermodynamicData["Water", "Density", {"Pressure" -> Quantity[1, "Atmosphere"],
"Temperature" -> Quantity[ThermodynamicData["Water","LiquidVaporPhaseBoundary",
{"Pressure" -> Quantity[1, "Atmosphere"]}][[1]], "Kelvin"]}]
Other than doing something ugly like this
ThermodynamicData["Water", "Density", {"Pressure" ->Quantity[1, "Atmosphere"],
"Temperature" -> Quantity[ThermodynamicData["Water","LiquidVaporPhaseBoundary",
{"Pressure" -> Quantity[1.001, "Atmosphere"]}][[1]], "Kelvin"]}]
how can I get the density of saturated vapor 958.3675 kg/m3?
Thanks.