Coming from a high level nordic combined (xc skiing and jumping) background and spending many years training full time, I can say that perhaps the greatest benefit of these new fitness tracking devices is that they measure heart rate variability (HRV). Following my nordic skiing career, my uncles and I (they were the good skiers, one a 3x olympian and the other a 1x olympian and NCAA champion) started a company to import and sell Firstbeat HRV software from Finland. All of the HRV in these wearable devices can be attributed to Firstbeat's research and analytics.
Anyway, I can see HRV being a valuable tool to water skiers. It is basically a measure of recovery but more specifically the body's return to homeostasis after stimulus. This stimulus can be exercise, diet, alcohol consumption, etc. High HRV indicates a recovered body, low HRV the opposite. If you have a couple beers at night, eat late, or workout late, HRV shows that in your sleep.
I'd encourage users of devices with HRV to pay attention to it daily and overtime- I bet there could be some really interesting takeaways for folks on here and could help optimize your ski schedule/training.