Which is not to say that building a German submarine may not pay off.
For instance, let’s pretend that England has Combined Arms and a large mass of destroyers. A solitary German submarine would deny every last destroyer and battleship from using their off shore bombardment privileges from SZ 5/SZ 14 which can save you large sums of money in potentially lost infantry.
For arguments sake, let’s pretend that England has 1 Battleship (left over from the first round of the game) + 7 Destroyers in SZ 5 + 1 Transport in SZ 5. That is a potential loss of 4 Infantry to Germany (25 punch / 6 = 4) it could be as bad as 8 infantry lost, or as good as none.
4 Infantry = 12 IPC worth of units and lost potential of having them already on the board.
1 Submarine = 8 IPC worth of units
It seems logical that the submarine would be the better choice, even though there is almost no chance it will survive to be used by Germany.