Halide perovskites are an emerging class of materials 1 of interest for optoelectronics and photovoltaics. Choosing different A-site cations, such as different organoammonium species, yields a wide range of structural and functional diversity beyond the prototypical "perovskite" structure. Halide perovskites are solution processable, which reduces manufacturing costs, and further capital cost reductions could be achieved by reducing the need for environmental controls during device fabrication. However, environmental parameters, such as humidity, remain an important and incompletely understood aspect of perovskite device fabrication. Water affects crystal growth kinetics, morphology, and stability, and is generally considered harmful to device stability and performance, although under certain conditions small amounts of water result in higher quality films and enhanced photovoltaic performances. This is attributed to water promoting earlier nucleation (which results in low nucleation density and larger grain sizes), whereas anhydrous conditions allow the system to reach a higher degree of supersaturation before nucleation occurs (resulting in increased nucleation site density and smaller grain sizes). Post-synthesis water treatment can reorient polycrystalline thin films, improving charge-carrier extraction in photovoltaic devices. However, research has focused primarily on methylammonium and formamidinium lead halides, and so less is known about the two-dimensional (Ruddlesden-Popper and/or Dion-Jacobsen) perovskite materials which have improved long-term photovoltaic stability. Single crystal studies provide insight into the growth process, as well as facilitating structure and property determinations for new materials. Inverse temperature crystallization (ITC) has many advantages for growing large, high-quality single crystals. ITC relies on a retrograde solubility effect, in which the product perovskite crystal is less soluble at high temperatures than the precursor species in solution. While previously believed to only occur for methylammonium and formamidinium perovskites, our recent work has found that ITC growth conditions can be used to produce 17 other lead iodide perovskites. Yet, the >1000 articles citing the seminal 2015 papers by Saidmanov et al. and Kadro et al. which introduced ITC growth for halide perovskites have paid scant attention to the role of water. Saidmanov et al.