Cognitive decline poses a major challenge in aging populations, and aquatic exercise may offer cognitive benefits while reduceding injury risk. This systematic review and network meta-analysis (NMA) evaluated the comparative effectiveness of different aquatic exercise modalities on cognitive function in older adults. Major electronic databases were searched from inception to January 2026 for randomized controlled trials (RCTs) examining aquatic exercise and cognitive outcomes in adults aged ≥60 years; studies were independently assessed by two reviewers. Interventions were categorized as aquatic cognitive-motor exercise, aquatic aerobic exercise, aquatic high-intensity interval training (HIIT), and aquatic strength training. A frequentist NMA was used to synthesize direct and indirect evidence, with effect sizes expressed as standardized mean differences (SMDs). Nine RCTs (involving 324 participants) met the inclusion criteria. Aquatic cognitive-motor exercise showed the highest probability of being the most effective intervention (SUCRA [surface under the cumulative ranking curve] = 85.36%), followed by aquatic aerobic exercise (61.44%) and aquatic strength training (51.80%). Aquatic HIIT (49.12%) and land-based exercise (48.34%) demonstrated moderate effects. Among all modalities only aquatic cognitive-motor exercise showed statistically significant improvements compared with non-exercising controls. Based on the currently available limited evidence, aquatic cognitive–motor exercise may be the most effective aquatic modality for improving cognitive function in older adults. All aquatic exercise modalities provided cognitive benefits at least comparable to those of land-based exercise. The findings suggest that cognitive engagement, rather than exercise intensity alone, may be crucial for maximizing the efficacy of the intervention. However, as only nine RCTs were included, these findings should be interpreted with caution given the limited size and density of the evidence network.