Yes, How can you work on the water supply if you don't understand what the law requires. (but sadly not many do have the ticket)
Only a plumber who has their water reg's ticket can issues a completion certificate (normally their bill) to the customer which means that the installation complies, otherwise if the installation / work breaks the Law they (the water user) could be prosecuted, not the installer.
It is regrettable that none are & so those who don't know what they are doing just carry on with immunity.