The bilge ports are painted at the whim of the bos'n mates who are down at the waterline doing the painting. If they feel artistic, they may take that extra little effort to make the demarkation line exact. If they are nursing a hangover and are bobbing around in a john boat with paint bucket and roller, they may just splatter the paint around just to make the chief think they've done something for the day. Hull painting isn't the same as painting an aircraft or a sports car and if you ever saw it up close you would notice all kinds of sloppy work.
As a side note: back in 1974, we pulled into Hong Kong and had some local painters come alongside and paint our hull while we went on liberty. (this was the USS Ranger CVA-61) We provided the paint and the Chinese provided the labor. They applied the paint with rags attached to poles and just slathered the hull like you would slop BBQ sauce on a rack of ribs. There was a slick of paint all around the ship for a hundred feet. What a mess! But the boat sure looked good from a distance. Accent on "distance".
Just paint your model the way you think it looks best, that's the way a sailor would do.