The Water Reservoir is a tricky one. Consider it very much a "get there, take the shot, get out" location, as any lingering over there is likely to get security personnel paying you a visit!
Access is on the right hand side of the treshold of Runway 24, but you will have to find a way in, as the fencing is often changed over there.
It is a great spot to photograph aircraft landing on Runway 24 in the afternoon, or airrcaft parked in Park 1 in the morning.