Brillouin spectroscopy provides a non-invasive, label-free method to evaluate the mechanical properties of biological materials. Prior studies have shown that the longitudinal elastic modulus, M, measured by Brillouin spectroscopy correlates with the Young’s modulus, E, of cells and tissues. However, M and E for hydrated materials are both influenced by water content. Using hydrogels as a simple model for hydrated biological materials, we designed experiments to separate the effects of E and water content on M. Polyethylene oxide (PEO) hydrogels were prepared with an average molecular weight of 1, 4 or 8 MDa and water content of 92, 95 or 98.5% (v/v). Polyacrylamide (PA) hydrogels were prepared with 10% acrylamide and 0.03-0.30% N-methylene-bis-acrylamide (w/v). E was measured by rheometry for PEO hydrogels and by unconfined compression for PA hydrogels. M was measured using a custom-built Brillouin spectrometer. For PEO hydrogels, E increased with molecular weight, whilst M was unaffected by molecular weight and decreased with increasing water content. For PA hydrogels, both M and E decreased over time due to swelling, but no single relationship could describe how M changed in terms of E. Regardless of swelling, all values of collapsed onto a single relationship that depended only on water content. After correcting for water content, measurements from Brillouin spectroscopy no longer correlate with Young’s modulus for hydrated materials. This work cautions against the straightforward application of Brillouin spectroscopy for optical elastography, but suggests that Brillouin spectroscopy and microscopy may be useful for investigating mechanisms involving changes in local water content.