I normally keep a 32oz narrow-mouth Nalgene bottle inside my HH with me. I simply hang it from the loop that connects the lid to the bottle by opening the bottle, putting the loop around the ridge line, and then screwing the cap back on. I can then slide it back and forth to get it out of my way.
I have thought about cutting a small slit in the netting right where it joins the hammock, near my head, and then hanging my water bladder on the outside of the hammock and then running the drinking tube inside the hammock. I would probably sew some velco on the hole so that I could close it up when not in use. Instant hammock hydration port!