Utility
As the Stones remind us, when we aim for utility we get not what we want, but what we need. Which, in a sane world, is what we should want. So the first question to ask when designing a waterspace is how it is going to be used, by whom, when, and what else will be going on around it. Time and again, when faced with the impenetrable task of designing a pool for a bunch of people I only met five minutes before, the solution became evident after getting the answers to the above questions. Of course, it also helped to take a good look around at the house to figure out what were the tastes of that family.
Another good design principle to apply is one I learned from an architect a long time ago, which is to treat outdoor space the same as indoor space. In other words, to look at it conceptually in terms of separate rooms. These outdoor virtual rooms will have different functions, will be of different sizes, and will have visual or physical separations (sometimes just suggested, with a planter box, or a cusp in the edge of the swimming pool, or a different pavement surface).