In our opinion, a swimming pool and spa designer is not only the one who can create the shape, but rather the one able to design how it should work and furthermore able to manage the building process.
We do prefer to do these  three tasks, but we are also called to do the two last ones, when the shape has been designed in a master plan or even the last one, if an engineering consultant did already the technical drawings. In this last case, our project management task is critical as we have to adapt the construction process to integrate efficiently the two other tasks made by others.