I have my own boat and hang around boatyards a lot, even travelling to places like Portsmouth Naval Yard, Philadelphia Naval Yard, etc. to look at ships and there's a couple of things I rarely, if ever see on models.
The first one is that almost ALL ships develop a scum of algae at the waterline and for the first 1/2 meter or so below the waterline. This is usually a dark green in colour, with hints of brown and perhaps a little tint of ochre. It's thickest right at the water edge and thins out as it goes down. It's also almost absent on parts that are not accesible to sunlight, such as underneath the transom at the stern. Sometimes, if a boat has been tied up to the dock for a long time (say more than a couple of months), you actually start to get long streamers of algae, almost like grass, wafting in the current.
Another thing is that there's almost always streaks of lighter colour going veritcally downwards. I don't know what causes it, it looks almost like bird poop but it can't be under water. Don't overdo it, it's a very faint colouring but it's almost always present and noticeable.
Finally, in salt water boats, especially those based in warmer water, there are barnacles. They're random and usually rough in texture. They're almost always white but sometimes have a grass green sort of colour on top where a different kind of algae has grown on them. Often they leave raised rings where the creature has been rubbed off.
