I just wanted to add that soulless and other archers will automatically fire at anything in the water regardless if it is visible or not. Therefore, the best thing to do before putting soulless in the water is to move them near by. If they shoot at the water you know 'something' is down there and that perhaps it is best to move along.

On the other hand, a nice trick is to place your own soulless over a pool with a wight in it. Should enemy soulless attack from high ground you can retreat. Often the enemy soulless will follow and then your booby trap is ready.