"An CG documented vessel is required to fly the 50 star flag not the yacht ensign. "
Sorry guys, both wrong. There's a good illustrated discussion at
A USCG documented pleasure craft is not *required* to fly any flag or ensign. It gains the optional *right* to fly the "yacht ensign", which is improper for an undocumented vessel. That's not the 50-star "union jack" either.
Since an "ensign" is defined as a "national flag" both the US flag and the "Yacht Ensign" may be called flags or ensigns, as you choose.
Apparently there has been some conflict over the two flags:
and some question and the current legal status of them, versus traditions.