Every country has its own rules. They come from international norms and from local ones. The same sheet of paper may pass on one shore and fail on another. We do not publish a table of “yes everywhere / no everywhere”: the desk and the law can diverge even inside one company.
Somewhere a boat certificate is not required. Example - Sweden: for many private outings a formal paper is not the gate it is on a Mediterranean charter. That does not mean there are no rules at all; it means the country’s picture is different.
Insurers may set extra requirements when the policy is written and when a claim is handled - separate from what harbour rules say.
If you sail your own yacht, under a given flag, in given waters, with given requirements and insurance - you need that whole link, not one certificate alone.
If you want a boat on charter without a skipper - better ask the charter company what paper they want at the base you chose. Or write to us: we will try to explain from the current situation. The situation can change year to year.
The path of skill on deck is on How to become a skipper. What an ISSA certificate confirms, and what it does not give you, is on certificates.