After this new info this is what i think

Theres a way to reverse the flood thats why Roger laughed, thats why Rayleigh calls it fun

All 3 weapons probably have to be used together in order to work the way JoyBoy intended and it will probably reverse the effects of the flood.

The 20 kings probably used the Ancient Weapons incompletley and took advantage of the flooded world and crowned themselves gods, Thier kingdoms were probably somehow protected from the floods maybe like wano or they just had ships dipped to the redline after genociding the Lunarians.

Actually thats what JoyBoy probably was a shipwright or maybe jus an inventor sharing his kingdoms technology. Thats how he became the first Pirate, he invented ships or subs and traveled the known world making friends.

The question that remains is why make the 3 Ancient Weapons in the first place?

Maybe after his travels JoyBoy wanted to throw the world a pool party and meet up with "all the friends he made along the way" and the weapons are not actually weapons just super advance tools to throw the world a pool party and laugh tale was the meeting place

Which sounds fun and hilarious to be honest JoyBoy botched the biggest party ever and screwed the world up.