The use of water as a solvent to facilitate supramolecular self-assembly and polymerization is well-documented; however, it is rare that water acts as a monomer that undergoes polymerization. We report the formation of nanosheets composed of water and a saddle-shaped porphyrinoid macrocycle, carpyridine, which allows for linearly stacked, eclipsed columns within formed 2D structures. Self-assembling carpyridine monomers from solutions with different extents of wetness permit the formation of nanosheets that appear identical by microscopy. Structural analysis through electron diffraction reveals fundamental changes in the local organization. Under dry conditions, carpyridine stacks are formed through π-π interactions between curved surfaces, whereas in solutions containing greater quantities of water, a hydrogen-bonded water-to-carpyridine-core network is propagated throughout perfectly linear columns. The observed wet phase can be interconverted to a dry one through vapor annealing, indicating an accessible energy surface of polymorphism.