The International Certificate of Competence (ICC) came about via UN Resolution 40 (UNR40), which was signed at various levels by some governments. Only agents of countries that have signed UNR40 can issue it. The United States is not a signatory to UNR40, so there is no US-based ICC agent. The ICC is exactly what it is named. It is mostly used around EU countries to certify that the charterer has the minimum skills necessary. Some countries require you to have the ICC before they will charter a yacht to you.
Not every country requires you to have an ICC. However, most charter companies will not charter a boat to you without an ICC, or an equivalent certificate. Since it isn’t easy for a US-based sailor to obtain an ICC, the ASA has an agreement with Mediterranean charter companies to provide a certification similar to the ICC called the International Proficiency Certificate (IPC).
Please note that any certification should also be paired with the appropriate sailing resume and you should check with your intended charter company to verify what they require.