[UKönig]

When using a U-boat equipped with a forward mounted deck gun, I like to copy the real world tactics. That is, start with Armour piercing, put a few pilot holes, and the follow it up with high explosive, into the same area.
The first shot gets the damage ball rolling, the HE chaser shot keeps it rolling.
Then I like to shoot at targets of opportunity, if I am reasonably close, like say 1km or less. At 2km to 5km (I try not to shoot at targets beyond the 5km range, unless I am trying to attract their attention), then I just aim for the biggest point I can hit. Which is usually the superstructure and the hull underneath same.
When time and range permit, I like to start with AP on the engine room, then switch to HE, and feed a few shots into their machinery spaces. Then I will move it forward and try puncturing the hull, above and at the waterline, to help encourage flooding with forward motion. Then I may move upstairs and attack their bridge, to put their helm/radio/command & control stations out of action. Can't send a distress call with no radio shack, so sometimes the bridge is the first place I will aim.
As far as ammunition rates go, the bigger the ship, the more you will have to throw. A 5" hole on a ship that is 8000 tons is still rather small and it may take quite a few hits before you see demonstrable results.
One thing a U-boat captain needs a lot of is...patience.