Dielectric relaxation of water for a wide range of temperature (373 K less than or equal to T less than or equal to 973 K) and density (0.07 g cm(-3)less than or equal to d less than or equal to 0.66 g cm(-3)) is studied with the molecular dynamics simulation technique. The evolving water electronic structure is explicitly incorporated via the truncated adiabatic basis-set representation. For a given density, polarization dynamics of water tends to become faster with increasing temperature. At reduced densities, it shows a marked deviation from a single exponential decay. Another interesting feature is that at fixed T, water dielectric relaxation becomes, in general, accelerated as the density decreases. This trend is at variance with recent microwave spectroscopy measurements at elevated temperatures and reduced densities. Possible sources for the discrepancy between the simulation and experimental results are discussed.