Roopak,
In case of deer, it becomes difficult for them to escape out of water once cornered. Once the spotted deer is driven into water, the dholes are all around the waterhole and close the exit for the deer. So the cheetal has to swim from one end to the other and get exhausted. It then becomes an easy prey. So as a strategy it works well for the dholes to force the spotted deer into water.

The height of the deer is less than than of Sambar. So where the deer gets tired due to swimming, the Sambar can stand due to its height. However, the dholes have to swim and lose energy while the Sambar is standing in the water. So in case of Sambar standing in water, the dholes normally don't enter into water. So in case a person is watching such a scene unfolding before him and the dholes give up the hunt, accurate observations will reveal that the dholes have done a realistic assessment and left the scene. They too need motivation and during chase one can often see them motivating each other.

Thanks for the local names. Any idea why the dholes are known as chennayi in tamil and malaylam?

We need to build up database of local names, else these will be long forgotten. I believe, when our languages lose words, the die a slow death and with the death of our languages a part of us die too.

Cheers,
Sabyasachi