Absolutely terrible.

While it was ridiculous that there was no spotter, the spotter probably wouldn't have help if the boat turned into the tube. That being said, had he a spotter, maybe he wouldn't have been looking back and would have been more aware of every boat around him.

Looks like so many mistakes all around. I would never pull a tube from a ski, without a spotter and on a busy day. Boaters look for other boats pulling skiers and tubes, not jet skis. They expect jet skis to be ripping around. That being said, the boat should always be aware well aware for 360* and for a great distance.
While the blame will probably be laid at the feet of the boater, on a non-legal level, the dad had a hand in this.