As of June 10, 2021, at least two-dozen countries have opened to U.S. travelers who are fully vaccinated against COVID-19.
The travel website AFAR is maintaining a one-page list of such nations at the following link:
That post includes at least two other links, both for U.S. State Department pages, that Seafarers may want to bookmark.
This one includes the latest travel guidance for U.S. citizens:
This one features country-specific information: