You're welcome Ernest.
Even though I live far from the ocean also, every chance I get I'm down at a dockyard somewhere. Portsmouth Navy Yard, Philadelphia Navy Yard, Groton, Norfolk, Halifax, and of course all sorts of civilian ports as well. One time I spent 10 fourteen hour days at the docks for a tall ship rendezvous. I think my wife was happy when the ships left.
And of course, even the lakes where I spend even more time, have weathering patterns to the shoreline, they're just a little different.
Back to the high water mark, in salt water, you'll see a distinct horizontal line at the point where the tide is highest. Above that line is discoloration, usually an olive green so dark it almost looks black, fading upwards for about 1/2 meter as the algae that creates it thins out. Since your dio has to be at high tide (there's not much seawall above the water, so it also has to be a very sheltered area, perhaps in a drydock) then that's what you need on the sides.
OIf it's in a dry dock, in addition to the rope fenders typically used by the Kriegsmarine, there might also be floating baulks between the boat and the wall, to protect the boat against abrassion. This was merely a big piece of wood, sometimes VERY aged, with holes drilled through each end so that it can be suspended by ropes to stay between the boat and the wall. Usually, it would be outboard of the fenders, so that the fenders would be what rubs against the wood.
As I said, your dio would pretty much have to be a dry dock for the water to be so high (waves would toss a boat OVER such a low seawall otherwise) and for the dock lines to be so short (not expecting waves or tide).
You could probably go online and find the typical tidal range for say Kiel or Hamburg, but I'd guess in that part of the world we'd be looking at around 6-10 feet. And that would be approx. every 12 hours.
Even on Lake Ontario, where my boat is, the water depth can change by up to 3 feet depending on which way the wind blows, so there always has to be slack in the dock lines.
There are particular ways of tying them up to build in that slack and still not drift too far from shore. Again, go to the internet, google "docking" and specifically look for what are called bow lines, stern lines and spring lines.
Nostly though, you'll see that a cleat or bollard on the boat is not directly across from the corresponding cleat or bollard on the dock. You want to have a sharp angle between them, so that the dock line is at an angle of perhaps 30 degrees. This allows movement with the waves. There's usually a minimum of 3 lines, 1 stern, 1 bow and one spring line. Most skippers will also put another spring line from the opposite end of the boat. sometimes criss-crossing in the middle if it's a small boat (but that wouldn't apply to a U-boat).
The bow and stern lines will probably run forward from the bow and backwards from the stern to again get that sharp angle to the dock.
If only I knew how to draw on this forum or attach drawings, I could show you. But the internet should have something under seamanship course.
