+1 on two key questions here:

1. Why did the camp allow such a weak swimmer on a high dive (or any board for that matter)? Lifeguards can't be responsible for that. I'd seriously question the supervision at this camp.

2. Why did your son think it was OK for him to go off a high dive when he essentially cannot swim? I'd have a major water safety talk with DS and let him know that there will be no more deep water without swimming lessons.

Our summer pool doesn't allow children to use the diving board if they haven't passed the swim test (swim a length of the pool and tread water for one minute).

There is risk associated with any activity. Presumably the summer camp took kid to pool with lifeguards who could rescue him if need be.

Sorry if I am missing something from your question.

Most camps have pretty strict requirements to use the deep end- either swimming a full lap, treading water, or both.

My DD does swim and water polo and ever pool we’ve ever visited has extensive signage saying a guard reserves the right to demand a swim test before kids use the board. One place she goes to requires a test + bracelet (like a stick-on wristband type one) to be worn to indicate that the swimmer is safe for the deep end. I think your kid got lucky in all ways and you need to practice more before he goes in the deep end or off a board again.

Did the summer camp do a swim test before letting kids go into the pool on their own? Most camps we've done have this.

I'd take him to the pool more and let him practice and get stronger!

Why is your older child such a poor swimmer? I’d worry about that first.

Yes, it is dangerous.

Diving always carries risks. But simple, more common difficulties (like landing wrong in the water or getting the wind knocked out of you) could be very different problems for a weak swimmer who might get anxious or struggle. A kid who can only "swim" far enough to get out of a diving well doesn't actually belong in water over their head.

My son can swim a little bit, like half lap. He told me that he jumped from diving board to 13 feet pool at a summer camp. They took kids to pool trip for a trip. The diving board was like probably 6 feet or so tall, and it was fun to spring up & down. He said that he wants to try cannolball one day.

Is it dangerous for not a good swimmer to try the diving board? He is almost 9.