Do like real life, you wouldnt know if there were any ships there! Prien didnt, he had info the whole of the North sea fleet was in dock, but the German spotter plane the days before had spooked them when he flew over too low, and ready for a bombing raid they sailed out.
Only the ageing Royal Oak stayed in dock (and a small aircraft carrier)
Thats part of the fun
the shock of discovery.
If you really want to know you can load up the mission editor and go to the date you are at and check, but thats cheating!!! :p