The exercise alone (no water immersion) improved V02 max better than 30 minutes of exercise followed by cold water immersion, but there were no significant differences between exercise alone and when an additional 30 minutes of exercise was substituted with 30 minutes of WARM water immersion.
Lest you think this is some standalone, unicorn experiment that no sober researchers will ever be able to duplicate, there’s a decent amount of already-completed related research that gives credence to the current study. At least two other studies (Gupte et al., 2009 and Sjoberg et al.) found that warm water immersion alone improved fasting glucose and glucose uptake, while a couple of others (Hoekstra, et al., 2018 and Green, et al., 2010) found that being a human teabag improved vascular function and structure.
The scientists involved in the cycling study summed it up this way:
“Collectively, these studies indicate that warm water immersion might be used as a viable substitute in achieving some of the benefits of exercise training (e.g., reduced risk of diabetes, improved heart health, improved cardiovascular fitness), particularly for those who cannot or will not exercise sufficiently.”
There’s also evidence, at least a little, that combining exercise and warm water immersion might also have some benefits to muscle and bone. A previous experiment (Tamura, et al., 2014) used such a protocol and found additive increases in both metabolic enzyme and protein adaptations in skeletal muscles, albeit those of mice.