Playing a game right now where, through smart unit cycling (i.e. pressing an entire Navy onto 1 unit at a time), the Central Powers player has decimated the British, French, and Italian navies. Subs have been absolutely the most brutal and most common enemy unit I've encountered as the Entente. Sometimes my destroyers will do 1 damage to a sub, if it can catch it. The problem is, once you try to chase the sub, you risk running into an ambush.
Really, unlike real life, the Germans have a good opportunity from the get-go to dominate the seas rather than keep a fleet in being, whereas the British should absolutely not attempt to leave the Channel and start a blockade unless you are sure you will smash any opposition. My opponent clearly understands this, as he has built up quite the sub fleet now.