Your assumption about the water here and 21 foot boats is completely off base.
That boat is fine for what they were doing...it is even rated for 10-12 people I think.
It is a good hull design and a very sea worthy boat.
They probably took a rogue wave from the fast moving squall line that came through unexpectedly and that is all it took.
Their big mistake was not having all the proper equipment for fishing that far off shore.
Contrary to what you may think people take fishing pretty seriously around here and drinking and drugs are very rare especially early in the AM.
What I read the first day is they were trying to pull up the anchor because the weather was so bad. If they were all on the one side of the boat trying to do this the boat would have been unbalanced and a rogue wave easily could have sent it turtle.