A ship/boat is cleaned below the waterline immediately when it comes out of the water. Where they haul my boat for the winter, the bottom is washed before it goes more than 10 metres (32 feet) from the water.

The reason is that once the hull dries, the algae and everything else is much harder to get off than when it's still wet. They use high-pressure water mixed with some kind of acid, I think muriatic acid but I'm not sure on that.

Now, to answer your other questions.

1) Grassweeds line is probably a local expression. I've heard it called a "grass skirt" but "grassweeds" is just as likely elsewhere.

2) It's possible but not likely that the algae at the waterline would be mistaken for a bootstripe. It's usually a blurry line, more distinct but still not straight at the actual waterline, and then fading out until it's gone about a foot below the surface. As far as an actual bootstripe goes, I've seen a picture showing Gunther Prien's U-47 with a fairly thick, slate coloured bootstripe above a red bottom. The bootstripe looked to be perhaps 3 feet in width on average but was wider over the tanks. My guess is that the bottom was red and the bootstripe was added later so that the red wouldn't show from above or when the wave troughs revealed the bottom.

3) Once the hull on my boat is washed, the weed line is the exact same colour as whatever paint had been under it. On the shaded part under the transom, there always seems to be a dark greenish-brown smudge where the algae is able to cling a bit better but that's about the only trace. Of course, if the bottom paint was old and most of the poisons had leached out already, then it becomes much harder to remove the growth.

4) If the bottom was not cleaned when it came out of the water, and the algae dried and got bleached by the sun, it might turn an off-white, parchment sort of colour. That would take a few months though I'd think and even then, there would be dirty brown colours showing in spots. It would show up white even more on a modern sub since the modern ones are usually all black topsides too. If the sub was still in the water, it wouldn't be salt because it would be washed off by the first wave that goes over it.

5) Yes, the algae can grow long if the ship is not cleaned for a long time and especially in tropical water and if it hasn't moved for a while. I've seen the weeds grow to perhaps a foot long, swirling around the hull and heard of ships where it has grown to a metre or maybe even more. The growth is quite uneven but tends to be longer the more sunlight can get to it so that the tanks would have longer growth than the part where the bow overhangs and shades it.

6) I'm not sure that the line would disappear if the water was changed but certainly the algae will die and fall off, leaving a dark residue behind. Very little salt water life can survive in fresh water and vice versa. Salt water barnacles will cling in fresh water even though they are dead but will eventually get knocked off leaving white rings.

7) I don't know but I don't think so. They will usually cling even if they are killed by the dive and new growth would occur before the dead stuff rotted off.

8) In two weeks, you wouldn't see any kind of a line at all, even if there was no poisonous bottom paint. It takes at least a month for it to even start feeling slimy (the first sign of growth) when the boat's not moving, longer if the boat is in motion.

Your summation of what would happen seems reasonable. If a boat is hauled out, as you noted before, the first thing they do is clean the bottom. However, the bottom and the topsides are not necessarily repainted. Bottom paint is very expensive and is not done more than once per year or even less frequently, depending on how much of the poison is left and how much the speed seems to be affected. Since you don't see the bottom, the purpose of the paint is preservative, not aesthetic even on a pleasure boat.

Painting topsides would not have been a priority unless rust bubbles (not just streaks of rust which is common) were showing right through the paint. There's many more urgent things that must be done on a sub than painting.

However, 15th Sept to 8th October is not very long to put a sub in drydock. Modern DE (diesel/electric) subs spend almost 6 months every time they go into drydock since subs have so many critical systems that must be worked on but of course in wartime, that may not have been the case.

Therfore, I'd be surprised a ship would be drydocked in that short a time. If there was damage, it would likely take longer than 3 weeks and if no damage, it seems unlikely it would have been drydocked other than for it's regular annual (modern navy) 6 month refit.

In 3 weeks, it's more likely to be just leave time for the crew, rearming, refueling and reprovisioning for the boat and a simple check of all systems. Three weeks sounds like just a regular turn-around time to me and not long enough to end up cleaning the hull.