Spanish Water Dog puppies should have their first haircut at 4-5 months, and then every 6-12 months after that, depending on personal preference.
If you want to maintain the easy-care, and rustic look of the SWD you will need to be clear with your groomer that you do not want a fancy look. Keep it simple. Same length all over. Any length is good as long as it is the same everywhere - including on top of the head, muzzle, and tail.
The coat should never be combed, brushed, or blow dried, and once long enough, cords can be pulled apart with your fingers. The coat will cord better the more the dog swims.