Yes, many of the freediving adaptions aren't of much use to UWH players. Of course experience in the water, finning technique, bouyancy control and the like are advantagious, but nothing that you can't learn playing hockey or training. The mental aspect and knowing how your body reacts in these situations is the main advantage, as well as an increased tolerance of raised CO2 levels in the body. That's why I suggested CO2 training above.

The interesting thing about the MDR and bloodshift is that past the point when it occurs, almost all the metabolism in the muscles goes to anaerobic lactic, which allows you to move in dynamics without increasing your heart rate or oxygen consumption. Of course this is of little use to UWH players, because when you surface after a few minutes of anaerobic lactic, you're stuffed!