I attached the top of a bottle to the "key" for winding a clock, hung the clock above my water container and string round the bottle top (as it unwinds with the clock running) drops a float that lets water out of the container. The way of attaching the bottle isn't ideal but the clock key has a left hand thread so it's really hard to fit anything else to it. I just drilled a hole in the bottle top, then carefully heated up the clock "Key" with a ciggy lighter, then pushed the key into the plastic to have a tight fit. Currently the string drops at about half an inch per hour. Wider "spool" for the string would mean faster string movement.
Please give it a try, thanks Brian. (You don't need to take the back off the clock, I just didn't know how they worked so I just wanted to look).