The linear modal and nonmodal stability of plane Poiseuille flow of supercritical water (SCW) with pronounced nonideal property variations is investigated. Strong variations in thermodynamic properties near the pseudocritical region are coupled using the approach developed by Ren et al. [J. Fluid Mech. 859, 89 (2019)] for supercritical ${\mathrm{CO}}_2$. The parameter space governing nonideal effects is spanned by the composite dimensionless parameter $\mathrm{PrEc}$, which characterizes viscous heating intensity, together with the thermal boundary condition $T_w^{*}$. Compared with previous results for supercritical ${\mathrm{CO}}_2$, SCW displays distinct modifications in neutral curve topology within the transcritical regime of nonideal parameter space, including a three-stage evolution of the envelope with increasing $\mathrm{PrEc}$ and the emergence of isolated island branches. In the sub- and supercritical regimes, the deviation of the two-dimensional critical Reynolds number from the isothermal value follows an approximate power-law dependence on $\mathrm{PrEc}$. Nonmodal analysis indicates that the classical small-$\alpha$ scaling between the rescaled maximum transient growth ${\overline{G}}_{max}$ and $(k^2,\alpha \mathrm{Re})$ remains applicable in nonideal regimes, and this behavior is examined over a wide Reynolds-number range. Transient energy budget analysis further reveals that nonideal effects do not alter the underlying shear-driven lift-up mechanism but redistribute energy among production, thermodynamic, and viscous channels. The results provide a systematic stability map for SCW channel flows and offer quantitative benchmarks for assessing nonideal effects in linear stability analyses of nonideal fluid systems.

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• Channel flow
• Flow instability
• Complex fluids