Upper panel: L_{H2O(321 − 312)} − L_TIR relation. The solid black line shows the best power-law fit from Yang et al. (2013) considering the low-z HII+mild and strong AGN sample (blue and red squares, respectively). The gray area is the 1σ confidence limit. The gold and magenta squares indicate the quasar host PJ231−20 and its companion, respectively. In the case of the companion, we report the cases assuming T_dust = 47 K (left half-filled magenta square) and 35 K (right half-filled magenta square). These points are connected by a dotted black line. Purple squares are the mid-z Hy/ULIRGs of Yang et al. (2016). We also report measurements of APM 08279+5255 (van der Werf et al. 2011), the z ∼ 6.3 SMG HFLS 3 (Riechers et al. 2013), and the z ∼ 6.5 J0439+1634 quasar (Yang et al. 2019a). Empty squares indicate measurements that were not corrected for gravitational lensing. Lower panel: L_H2O/L_TIR vs. total IR luminosity. Symbols are the same as in the upper panel. Together with L_H2O/L_TIR for the 3₂₁ − 3₁₂ transition, we also report those for 3₁₂ − 2₂₁ and 3₀₃ − 2₁₂ for QSO PJ231−20, the companion galaxy, QSO PJ308−21, and the quasar J0439+1634, as indicated in the legend.