Background: Thermoneutral head-out water immersion is used as a ground-based microgravity analog, but prefrontal hemodynamic responses to postural transitions in water are poorly characterized. This pilot study tested whether immersion attenuates the post-standing oxygenated hemoglobin (O₂Hb) reduction relative to land. Methods: Bilateral prefrontal O₂Hb and deoxygenated hemoglobin (HHb) were recorded with functional near-infrared spectroscopy (fNIRS) in eight healthy adults (22.8 ± 4.9 years; 4 males, 4 females) during counterbalanced, within-subject supine-to-stand transitions in water and on land. Responses were analysed in three 30-s epochs relative to condition-specific supine baselines. Conditions were compared using two-sided paired-samples t tests; exploratory one-sample t tests examined within-condition changes, with Cohen’s dz as the effect size. Results: No between-condition comparison was statistically significant. The largest mean difference occurred for O₂Hb at 60–90 s (land − water = − 1.87 µmol/L, 95% CI [− 4.19, 0.45]; p = 0.098; dᶻ = −0.68). Exploratory analyses showed O₂Hb significantly below baseline on land at all three epochs (p ≤ 0.010), whereas the reduction in water was not statistically significant (p = 0.086–0.139). HHb did not differ from baseline or between conditions (p ≥ 0.324). Conclusions: The primary analysis did not support the hypothesis that immersion attenuates the post-standing O₂Hb reduction. Although mean O₂Hb reductions were numerically greater on land than in water, no between-condition comparison reached statistical significance, and this within-condition pattern does not establish a difference between environments. Given the small pilot sample, these null findings do not establish equivalence, and they do not support a microgravity analog effect.