Naval defense 0 lets all types of warships gang up on an isolated sub for free. Typically in this game you want to hit the sub with your non-damaging warships first (battleships, cruisers) and then finish with destroyers once the sub has been attacked down to a low supply value. This ensures that the destroyer attacks do high strength damage.
If the sub had defense of 1 or greater, battleships and armored cruisers would take damage for participating. But as of now there's no downside to using dreadnoughts to attack subs.
When a sub dives it teleports to a nearby hex and possibly takes damage (and I think it still loses a supply point). This can save the sub sometimes in edge cases, but it's less effective when the enemy has half their fleet focusing on one sub, because there are plenty of search vessels around. Even if a sub does survive due to diving one or more times during a single turn, it's often going to be destroyed during the next turn because it has low supply/movement points left and enemy warships stand between the sub and escape.
I haven't played the WWII games so I'm not sure how subs are successful in those games.
We can test out to see if higher dive percentage does anything, but in my opinion it probably won't change much.
I was trying to come up with a way to improve the situation and one thing I did was give subs 1 defense and destroyers +1 sub attack. (Could possibly give only upgraded subs this defense bonus). This isn't a perfect fix at all, but it at least makes it costly for dreadnoughts to participate in the feeding frenzy.
Yes, pretty much that. Ideally I think it should be balanced in a way that say 3-4 DDs are a serious threat to a lone sub, but a battlefleet with 2 DDs is not.
Btw, contrary to the threads title, I think subs are really strong in this game, just not in their intended role .
Have a skilled Naval player in charge of CP and thanks to the subs super stealth scouting ability the Royal Navy has to run and hide from the Hochseeflotte the first few turns.