The main issues you are likely to have with using just one vast water plane is when you need to create dry areas that are below this level. For instance, cutting bunkers can often be a problem if the land is just above water level, and land fractionally above the water plane can sometimes cause problems in PG where the ball still registers as hitting water when it actually hits land fractionally above the plane. Apart from these design problems I don't think there is any technical reason that you can't have one water plane covering your entire plot. In fact, I may have done this with Abbotsley, I can't remember. I know I did initially, but I may have later split up the water into several planes.