Java implementation of web services is built on top of either
servlet or
ejb. You can use either one, to implement your web service.
In web services, you will find it easy to understand the servlet implementation/relationship part, as you have scwcd. But the exam also includes the ejb implementation. So it can help if you also know ejb.
But you dont need to have scbcd level of knowledge to understand web services ejb implementation. I think if you know the session beans and how the ejb container manages it, you can also take scdjws.