CEVNI stands for Code Européen des Voies de Navigation Intérieure. If you plan to use a boat on the inland waterways of mainland Europe then you probably need both an ICC and a CEVNI endorsement (N.B. you cannot have a CEVNI endorsement without first obtaining an ICC).
How to get the CEVNI endorsement added to your ICC is quite straightforward. There is a short test which takes between 15 & 30 minutes involving a series of multiple choice questions. There is no course for this, just a book of rules & regulations which you need to learn before coming to take your test. We have copies of the RYA Inland Waterways Regulations in stock, and you can book a test at a time to suit you. The best explanation of CEVNI requirements is HERE