It can also be neither. A Notified Body is not required if the boat is below a certain size and designed to be used in Category C or D conditions.
A builder has to create a Technicl Construction File for the boat and build the boat to meet all the current ISO Standards that are relevent to his product. As long as he does this then he can self certify and put the CE mark on the finished product.
If he builds a boat that is designed to be used in Category B then certain elements of the build have to be certified by a Notified Body. there are about 30 Notified Bodies in Europe. Any manufacturer, anywhere in the world, may choose to use any of the European Notified Bodies. There are two in the UK. The RYA is one, the other is HPi Verification in Oxford.