OK
1.TO lock a target go to your periscope and hover the target cross on a ship and hit the spacebar
2.Enemies of Germany I dont know what you mean. someone else may help you
3.To order a specific depth press your Ctrl button on your keyboard and the depth selection will go advanced so you can order a specific depth
4.Ship Icons some mods in this case NewUIs by TDW remove the vanilla effect of the speed of a ship on the map when you click it
5.So the deck gun or flak gun returns to the original position secure from battle stations
6.The crew vanish because you are underwater when the waves hit dont worry your crew is not overboard they will come back.
7.The purpose is to keep other stations dry if there is a flood in another compartment. You cant dive until you are off the bridge and in to the conning room, the top hatch will close automatically when you hit any dive button. Plus you'll have to manually close each hatch by going near them and clicking on them if you want to but this is not worth it.
8.DDs not attacking you in Scapa is a known bug
Good Luck Happy hunting
