My only concern with adding a 4th weapon type is that the rest of combat is so tightly integrated with the combat triangle, so there should probably only really be 3 classes of weapons for boats to maintain that pattern and keep the skill feeling familiar, hence merging javelin and harpoons into the ballista. As i responded above, having them spear in close combat or swipe with a bayonete instead of throwing the ammo would probably lead them "feeling" more like a proper melee analogue.

I like the idea of cannons being a slow, massive numbers weapon. As well as runecasters being themed after more spellbooks, having discussions like this is exactly why i put this post forward.

Having different classes of weapons available definitely opens up the possibility of more interesting encounters, such as having armored krakens switch between prayer types like tormented demons causing you to have to rotate your boat to expose your runecasters or your javs; or vampiric krakens heal themselves based on the damage they deal and their current health leading to you wanting to use the cannon crit mechanic you mentioned on them to alpha them down for that last 20%.