I still think that if the system is properly balanced, then a captain playing with 100% realism should/will have very little global influence, aside from local/tactical changes. Well assuming the game doesn't let you sink 5 million tons of shipping with "100% realism"
Even for the "remarkable" events, say you save the Bismack, in the end Germany still loses air superiority (nothing a sub can do about that) and odds are that the Bismark will just end up being a Tirpitz bis so ultimately it's not really a big deal.
I don't remember where it was, but a dev said that your actions can also trigger Allied reaction, for example sinking a carrier in a convoy will prompt them to stop doing that. Unlike SH3 where this is all scripted so you can sink a lot of battleships and carrier in convoys until they stop being scripted that way.