Wilber et al. 1996, Influence of water run training on the maintenance of aerobic performance

Sixteen trained male runners were split between six weeks of deep-water running and six weeks of land running at matched intensity; no significant within- or between-group change in treadmill VO₂max was found, so deep-water running maintained aerobic capacity as well as continued land running. Med Sci Sports Exerc 28(8):1056-62. Small sample, short term, and intensity was deliberately held high, so the result is maintenance, not improvement. Moderate.