Here's the problem: two of the six holes that I've dug so far have hit water at 20-24" down. I hit basically large natural gravel if I dig further. One of these water holes is on a corner.
So, how would you approach? Would you set the post and just use gravel packed around it? Would you throw some gravel into the hole until the waterline disappears, then set post a bit less buried than you were planning (making the post shorter to compensate) and fill with concrete? Would you throw a bag of concrete into the water and set the post on top of that? Would you go ahead and set the post as you originally planned?
If this was a fence, I wouldn't worry so much. Instead it's a significantly large coop that I designed so I can add onto it later, so I wanted to build it pretty stable this go-round!