Double sideband water line spectra toward W51. The top row shows the excited transitions of H ₂O, the middle row shows the ground transitions of H ₂O, and the bottom row shows the ground transitions of H ₂ ¹⁸O. In each panel, we show the H polarization spectrum (black solid line), the interpolated continuum (blue solid line), the best-fit continuum (orange solid line), the emission components (orange dashed lines), the best fit (red solid line), the absorption components (red dashed lines), and the zero continuum level for the single sideband (gray dashed line). Some of the features seen are not due to water molecules (see the text for details). For instance, the 45 km s ⁻¹ emission feature at 547 GHz is likely due to ³⁴SO ₂ and methanol. The 40 km s ⁻¹ absorption feature at 1655 GHz is due to the strongest water absorption at 1661 GHz. The 35 and 50 km s ⁻¹ absorption features at 1661 GHz are likely due to CH.