I have found the use of submarines in Global pretty underwhelming. While the Axis don’t need any help in the OOB game, I did want something that would provide the desired flavor, within the scope of A&A (and understandably balanced by some benefit to the Allies).
German Submarines (U-boats)
German submarines, commonly known as u-boats, benefited over their opponents with advanced tactics. Except the characteristics above and where mentioned below, they are treated as submarines for all other purposes, including Surprise Strike, Submersible, On Station, Treat Hostile Sea Zones as Friendly, Doesn’t Block Enemy Movement, Can’t Hit Air Units, and Can’t Be Hit by Air Units.
Limited Nationality: Only Germany can purchase German submarines.
Wolf Pack: German submarines can be combined with other German submarines. A matched pair of two German submarines increases the attack value of both submarines from 2 to 3.
Condor Spotting: Strategic bombers can be combined with German submarines. A matched pair of a strategic bomber and a German submarine increases the submarine’s attack value from 2 to 3.