Background: Competitive sports represent a powerful physiological and psychological stressor capable of modulating neuroendocrine and immune pathways. Water polo, characterized by intense intermittent exertion and frequent physical contact, provides a unique model to investigate competition-related stress biology. Methods: Sixteen male Italian Serie C water polo players were enrolled in the study. Using a within-subject design, saliva samples were collected under controlled circadian conditions. Salivary biomarkers, including cortisol, IgA, and cytokines, were assessed both before and after training sessions and competitive matches. Results: Both training and competition elicited POST-session increases in salivary cortisol and cytokines, alongside reductions in IgA. However, competition produced significantly higher anticipatory and POST-session cortisol concentrations. A larger POST-session decreases in IgA compared with training was observed. Cytokine concentrations increased from PRE- to POST-session in both conditions, with significantly greater induction during competition across the panel. During training, selected cytokines showed positive within-session correlations with cortisol, indicating coordinated hypothalamic–pituitary–adrenal–immune activation under lower psychosocial load. These associations were attenuated and less consistent during competition. Conclusions: Official competition amplifies endocrine and immune responses beyond those observed during match-like training in elite water polo players, despite comparable physical demands. Altered cytokine–cortisol coupling under competitive conditions suggests modulation of neuroendocrine–immune integration by psychosocial stress. Combined salivary profiling of cortisol, cytokines, and IgA represents a feasible, non-invasive approach for monitoring psychophysiological load in elite aquatic team sports.