I looked over the Web site selling the competency certs. There is enough equivocal language on the site to indicate it's not an official, government-recognized or government-required document. While it may soothe a cruiser's worries, to me it represents and outflow of cash for an indeterminate benefit.
There are enough cruisers out there who have the sea miles without any certificates, and they are happily cruising. If you want a cert, take courses from US Sailing,
ASA, or your local US
Power Squadron. They all offer hands-on courses with
boats and you can get
documentation establishing your course level and competence.
People have successfully used these to
charter boats in
Europe without requiring another cert, costing more
money, from one of the many European authorities. It's like the reciprocity with your US driver's
license -- you can use it to rent a car and drive it; you don't need to get an EU-issued one.
Are there exceptions? Probably. Check with a cruising guide for destination-specific requirements.
You can also fabricate your own beribboned, fancy-bordered certificate, official-looking enough to impress even the most jaded harbor master.