It makes sense. Maybe Pluton was simply Joy Boy's super giant pirate ship which needed to be stupidly huge, powerful and durable to carry his gigantic robot and elephant buddies around, along with whatever else he had on his crew. Shirahoshi's ancestress may have just been a giant mermaid who was a member of his crew too. And whatever Uranus is, it's likely something Joy Boy cared about too but because of its huge size or strength, it’s treated as a weapon.
I feel like Joyboys dream was simply to allow the people of the world to freely sail the seas as such the three great weapons were designed to allow that:
1. Poseidon to calm the sea kings so that there’s no danger from them
2. Pluton to carve a hole through the red line so that the seas can be connected
3. Uranus to pull the rubble from the hole into the White Sea above
However, the twenty kingdoms saw these could be weapons. If Pluton can break the red line, why can’t it break an island. If Uranus can pull something into to the White Sea, why can’t it push something to the bottom of the blue sea.
Shit like this. It’s like Tom said, a ship isn’t good or bad. It’s whoever uses it