Hello Siara,
Last lot of photos. I took these at a few local marinas. I should point out that the water in these marinas was MUCH cleaner than the water in U-boat ports - hence no scumline. But they do show the colour of the minor plantlife at the waterline. The first two boats haven't been in the water for long -
This one shows dirt from the dirty water in the marina -
These two have been in the water for much longer -
I hope all these images are useful. But even with them it is still difficult to gauge what U 552 would have looked like below the waterline.
The scumline you've recently added is excellent. It correlates with U 552 at the time you are doing the boat. You could get away with a minor bit of plantlife at the bottom of the scumline. The lower hull might look a bit messier than yours is at the moment. I guess it might be hard for you to mess up your pristine looking model! You know the series of photos in Steve Wiper's VII pictorial showing U 99 in dry-dock? Parts of the U 99 lower hull are very patchy. Again, I'm not saying that your U 552 would have been as patchy as U 99 (U 99 may have been at sea longer). But a number of dry-dock shots (U 453 in U-Boot im Focus 3 is a good example) show U-boat lower hulls looking very patchy. Wish they were in colour!
I wish I could help you more with exactly what U 552 would have looked like below the waterline. To be entirely frank, I'm not sure myself - just doing what I can to help you and others. As I said earlier, I reckon this part of your U 552 is the most challenging. With your skills you can find out what parts looked like and just build them. But the lower hull weathering is, to many of us, a largely unanswered subject - and certainly a subjective one. At least you have a great degree of artictic licence here.
Cheers,
Dougie