@Gliblord:
The horrid aim of enemy cannoneers in the series is really stupid. I get that the effect of the cannonballs causing gushes of water is dramatic and all, but if that World Noble could take out Sabo's tiny fishing boat from atop a huge-ass ship then trained cannoneers ought to be able to hit a boat nine times out of ten–especially if they're level with each other and close-by. Though I guess you could say Luffy would just repel any cannonballs that would damage his ship anyway, so might as well make for drama.
Or maybe it's like how antique pistols have shitty accuracy, and the technology of the cannons is not quite up to speed with, say, Vegapunk's laser beams?
Well, you're talking about two things, weapon precision and shooter accuracy. The precision of the weapon (I fire it in the same exact position 100 times, how close together do they hit) isn't a factor in a manga world I don't think. Otherwise, people like Usopp couldn't be as awesome as they are without relying heavily on luck.
As far as shooter accuracy goes, it should be typical for the first shot from a cannon to be off the mark. It's very difficult to account for all the factors. After seeing where it lands, adjustments can be made and the second shot should be far more accurate. This holds fairly true in the manga, I think. It's only in the anime where it gets drawn out and that feeling becomes warped, IMO.