The ICC is required by many European countries for certain categories of pleasure boat. It is also useful in countries where compulsory certificates are required of nationals simply because officials expect to see such a certificate.
Applicants requiring a certificate valid for inland waters must pass a written test on the CEVNI rules (European Inland Waterways Regulations).
The requirements for certificates of competence vary from country to country. Sometimes they are required for coastal waters, sometimes for inland waters, sometimes for neither and sometimes for both. As the skipper of a vessel, you must ensure that you area aware of any requirement for qualifications before venturing into another country's jurisdiction.
In very general terms an ICC is required for the inland waterways of Europe and for inland and coastal waters of Mediterranean countries. For the coastal waters of Northern Europe the ICC is generally not required, however to all of these generalisations there are exceptions.