Hint 1.8b
The pressure you have calculated is the gauge pressure. A negative gauge pressure means that the pressure is less than atmospheric pressure. This makes sense because the the pressure at the bottom of the pipe is atmospheric pressure, and the pressure decreases as you go up the pipe. (Since the velocity of the water is the same everywhere in the pipe, the pressure doesn't change because of a change in velocity.)
Note, however, that negative true pressure is impossible.