I've cooked a lot of chicken w/o a water pan and didn't have any issues. You will get a bit of smoke from the drippings vaporizing when they hit the coals, but I haven't noticed it causing an off taste.
On a side note, I think the whole beer can chicken thing is just a gimmick. The fluid in the can doesn't get hot enough to evaporate and flavor the chicken, at least it didn't the few times I tried it a long time ago. What I like to do is insert the wire stand that the beer can sets in into the chicken empty without a can of anything, it holds the cavity open so the heat and smoke can get inside the bird.