Not sure waht the requirements are in Th US but if you plan to charter in Europe, Medditerrain etc..
These are the basic requirements to skipper a bareboat.
You must have a Captains license,ASA course 104,103 and 101. or equilent RYA day skipper ticket.
Charter compaines do not legally allow you to sail at night, so forget that option.
HAving said all that I have seen too many fake licenses made on computers and accepted by charter companies as real. If a charter company lets you take a yacht out without the proper qualification then you are asking for trouble, damaging a yacht is expensive and costly.
An alternative Idea is to get your ticket while on vacation, there are many schools that will teach you sailing and get you though the basic course so that you can later charter your own yacht.
Go and look at this site
they are a fully licensed school teaching ASA,RYA and IMT licenses to clients in the greek islands