We present an accurate equation of state for water based on a simple microscopic Hamiltonian, with only four parameters that are well-constrained by bulk experimental data. With one additional parameter for the range of interaction, this model yields a computationally efficient free-energy functional for inhomogeneous water, which captures short-ranged correlations, cavitation energies, and, with suitable long-range corrections, the nonlinear dielectric response of water, making it an excellent candidate for the studies of mesoscale water and for use in ab initio solvation methods.