All submarines do Convoy Raid Damage, German Submarines do it more effectively.
Germany does 3 CRD
USA, Australia, England (And India), Italy, Japan and Russia do 2 CRD
China does 0 CRD (cannot buy submarines)
It would take 3 Japanese submarines to do 6 dmg to England, but it only takes 2 Germany submarines to do 6 dmg to England.