You really don't need to add water , moisture from the ground will suffice, did when they built my pole barn.....Put some in the hole first then set the posts, or drive heavy nails part way in the posts.I once built my deck by putting concrete around the posts and the snow load caused the deck to settle a little, had to jack it up leveland set blocks under it....because I didn't have a solid bottom..
Concrete mix just dumped in the hole will absorb water and get firm, but not as solid as mixed and poured. I have hauled bags of concrete mix and buckets of water up hills and down to pour for corner posts for high tensil fence.
I mixed it in a 5 gal plastic bucket about a third of a bag at a time, The first third went down first around the post. Then a little dirt and rocks and then the rest of the mixed concrete and filled the rest with tamped dirt and rocks. The posts haven't yet leaned under the tension of the fence.
When my pole barn was built the contractor had 8 inch round x 3 inch brick like things that he dropped down the holes before setting the poles. It was to keep the poles from settling downward. I have been thinking about building a deer tower and would set the poles the same way.
I've done it both ways....If I'm planting a pole I usually just dump it in dry as others have said. If I want a concrete pad at the bottom of a hole, I dump the concrete mix in add water and mix it up with the post hole digger and let dry.
We replaced some treated 4X4 posts on our discgolf course by just pouring sakrete around the posts and pouring a bucket of water on top. The weight of the concrete held the posts level until it set up.
I work with concrete almost daily as a construction inspector and have ran a two bridge project from the ground up so ill probly over think this but first off-
1. don't spray the metal that will be in the concrete with anything its a bond breaker and can result the concrete breaking loose an allowing water to seep into the cracks.
2. while you can just dump the bags into the ground an allow the moisture to slowly seep into the crete this depending on the size of your hole may end up not fully curing if say the exterior gets moist then hardens the interior where the pole is my never receive moisture you are better off mixing water, as concrete takes up to 30days before hardening completely depending on the size of the pour (the hoover dam is still curing) id suggest 6-12" around the pole for a 4x4" that will be fine.
3. to make the pour easier id go with 40lb bags mix one at a time it doesnt take much water and 40lb can be a handful depending on the person. be sure to use a spade shovel to consolidate concrete in the hole around the poles for a better end product the less air in the concrete the better off you are.
with that all being said depending on what your pouring you may only need a couple of bags from the lowes just make sure it has aggregate(stone) in the mix you dont want a mortar mix without any aggregate it will have no structure strength.