Several highly accurate formulations of the thermodynamic properties of water and steam are investigated in the regions of supercooled, superheated and stretched liquid water. It is shown that they give considerably different locations for the spinodal, which diminishes the physical relevance of this curve. It is argued that nucleation theories based on local equilibrium fail long before the spinodal is reached. We calculate the physical limit of metastability for superheated and stretched water based on the theory of relaxation of metastable states of Patashinskii and Shumilo. For this application a crossover formalism is used to enable calculation of the critical radius of the nucleus for both positive and negative pressures. The calculated physical limit of metastability agrees with the extrema of superheat, realized in homogeneous states by Skripov et al., and of negative pressure, realized in homogeneous states by Zheng et al.