<p>Postpartum depression is a significant public health problem affecting both mothers and infants and is associated with obstetric, psychological, and social factors. Although water birth is considered to facilitate labor and improve maternal comfort, its relationship with postpartum depression remains unclear. This prospective observational cohort study compared depressive symptom scores between women who underwent water birth and those who had conventional vaginal birth on land while evaluating the role of partner support. A total of 87 women were followed from the third trimester of pregnancy to the third postpartum month. Depressive symptoms were assessed using the Edinburgh Postnatal Depression Scale (EPDS), and perceived partner support was evaluated using the partner subscale of the Multidimensional Scale of Perceived Social Support (MSPSS). Postpartum depressive symptom levels were lower in the water birth group, particularly with higher partner support. EPDS scores changed significantly over time (<i>p</i> &lt; 0.001), and symptom trajectories differed between delivery groups (<i>p</i> = 0.037). Adjusted mean EPDS scores were modestly but significantly lower in the water birth group (<i>p</i> = 0.038), while partner support demonstrated a strong and consistent independent association with lower depressive symptom scores throughout the study period (<i>p</i> &lt; 0.001). These findings suggest that partner support is the key predictor of postpartum depressive symptoms and that any potential benefit of water birth is likely context-dependent on the presence of adequate psychosocial support, particularly partner support.</p>