You don't need to replace the water pump and cambelt together - but you can save yourself time and money if you do so. They are both very expensive repairs on their own, because they are so difficult to access. However, if a technician is already replacing your cambelt, it won't take them much longer to replace the water pump as well. That's why it's worthwhile replacing both at the same time.
Plus, they both have similar lifespans, so replacing them at the same time can benefit your car's performance.