Dropping the ice directly in to the water generally drops the water temp more than I desire unless you limit the amount of ice thrown in. Putting in small amounts of ice multiple times in the day to keep a more constant water temp is not very convenient.
Having the ice in a tray above the water line melts the ice at a slower rate. This will cool the water at a more consistent rate. This method works well for keeping bait alive overnight in hot weather. Generally a bottle of ice in the tray in the morning and once again in the evening keeps bait alive for several days while not fishing.