Welcome to the real world...
The most difficult to appreciate is the distance (specially when sea is a bit rough), which can induce errors in the target course. Don't hesitate to make several plottings in sequence 3'15", 6'30", 9'45" and repeat. The more estimated plots you have, the easiest it it to find an average course and speed. Also like Codmander said, the closest you can get for shooting (I try to be between 500 and 800m) the more forgiving will be your estimations. You will end up not hitting at the precise spot if you have a speed error for instance, at a longer distance it would end up by a miss.
Don't forget to open tube doors before firing and try to fire with a gyroangle as close to 000 as possible.
Gute Jagd !