You’re correct about this. I was just looking it up myself.

There is indeed international maritime law governing vessels, and they must proceed with all possible haste to distress signals. Unfortunately, there are two problems here as I understand the situation:

1. Believe it if you want to or don’t but - the people operating Zuckerberg’s yacht claim they hadn’t heard or noticed the distress signal
2. The Coast Guard - who, importantly, the stranded skiff had initially contacted - didn’t consider their situation to be worthy of “distress” and thus Zuckerberg and his yachtspeople didn’t violate any regulations or laws.