First off remember to lead you target. Depending on you position and range in relation to the target and your relative speed you may have to lead by alot or very little.
Second and most important factor is the state of the sea. No swell, little swell, heavy swell make all the difference and you have to compensate for it.
In the Marines I would bet you were shooting from a ground location. Now you are shooting from the deck of a Uboat. Whole new ball game shooting from a very unstable platform. There are 3 deck gun views regular (can't see s@@@ unless you are close say 500m. 1st zoom level can see and target up to 3000m. Extra zoom can actually see and target up to 6-7km.
We are interested in the second setting. Why you ask? Simply because in this one you can see your gun bore on the left side of you screen going up and down as your Uboat heaves up and down with the relentless waves.
That's right you guessed it you must time you shots when the barrel of the gun is more or less centered in the horizontal position and not when its pointing at the moon or aiming at the fish.
The bigger the swell of the waves the harder it is to shoot. The more off the center-line of the Uboat the deck gun is turned the more you feel the unstabilizing effect of the swell.
Patience is the key word here. Wait for the right moment to fire your shots and eventually you get in sync with the motion of the swells and hit the bulls-eye.
That's all you need to know now go out there and practice what you have learned
P.S. In moderate to heavy swell never let your drunken sailors fire the deck gun no matter how may qualifications they have. All you are doing is wasting you ammo. Fire manually you chances of success are far greater.
P.P.S. As the war advances this becomes more of a challenge, the merchies start firing back with better and more numerous guns as the war progresses. They also call in avaialble air support or surface ships in the area (50 km radius).