It seems that the top of the booster fell off when the booster tipped over after a wave hit the droneship. Hopefully not typical.
The bottom portion (RP-1 tank), the landing legs, and the engines stayed on the ship. Octagrabber sustained some damage.
Photos from various Florida Spaceport photographers:
Videos from NSF Space Coast Live:
It was. RIP B1058. http://www.collectspace.com/news/news-122623a-spacex-falcon-9-first-stage-1058-lost-recovery.html