[Eru]

where can you get these cannons that does aoe dmg? and is it like a weapon that goes into your backpack?

Can't get them. You can see some working ones in East Britain in front of the Tortuga(?) Bar. (The one with the giant ship's sail.) There are purchasable cannonballs and I think it takes sulfurous ash also. The damage is very minimal, but it could add up if you have 20+ provoked sea serpents in an area like I used to.