When smoking a turkey, it's not necessary to use a water pan, though keeping a pan full of water in the smoker does have benefits at certain times.
In cold weather, a waterpan filled with boiling hot water can help maintain the smoker temperature, plus it gets the temperature up quickly for a fast start. This is especially helpful if your smoker can't quite do the job when it's cold outside.
When using a vertical smoker, you'll have to use a pan of some kind under the turkey to prevent drippings from falling on the hot electric element or charcoal. You'll have flareups galore, and that's NOT a good thing!
Using the water pan will prevent flareups, since it will catch the drips. However the pan of water will also cause the smoke chamber humidity to be high, which will give the turkey rubbery skin.
To solve this problem in a vertical smoker, you can place the turkey on a raised rack, and place the rack in a baking pan. That'll catch the juices, and still allow the turkey to be smoked as usual.