I have no real knowledge about this, I've just seen them work at the underwater treadmill and thought I would like to use that as well ;-)
I think the extra endurance without much stress on the bones is of course good, but what I meant above was more the effect of fast movement with the legs under water. Not so much for endurance but more like for recovery of the legs. So I imagine them doing a hard workout in the morning the treadmill in the evening and then the legs feel fresh again, for what you would need a day or two days of normally. So they can go on the track or for a tough tempo right on the next day again.
Jim Ryun also used to kick his legs lose in the pool every day.