Hi
Here's a link to the qualifications that are acceptable for chartering a Croatian flagged boat in Croatia... https://mmpi.gov.hr/UserDocsImages/dokumenti/MORE/More%207_19/TABLICA_ENG_srpanj2019%205-7_19.pdf . USA Qualifications that are acceptable are on page 17 and 18.

The best month to go if you are not constrained by kids is September. Warmest seas and still, usually, good weather and less crowded than July and August. Naturally all Europeans also know this is the best month to go so book early if you find a boat you like.