@Felius
Add some AA to the SHBB. The +2% IC is bad, but not that bad, specially given that its biggest weakness are going to be air attacks (so long it doesn't lose its screens). The 10% extra AA from Anti Air Battery I makes quite a bit of difference. Dropping some of the big guns can be advantageous too, not because of cost savings but because of speed gains. And if you are building SHBB you probably should have more of everything else too, so the damage loss is not going to be that critical. Their biggest advantage is their piercing capability, to defeat other heavily armored ships.
SHBB already have 10 AA in the picture, remember im using 4
Double Purpose guns. the optimum value is 5AA (-15% dmg). Going above 10 is useless as you need a exponentially higher value to get +1% benefit. No point going to 15 or 20AA and getting multiple times the +2% cost penalty.
You can see more about AAhere
About SHBB´s no matter how many guns you remove, you are slow. Around 30knots is the best you can go with 1-2 guns and thats is too low for such a expensive ship.
At best you might want to get to 25knots but for a SHBB usually you have HP to escape even if you're slow as you can take some critical hits. (Critical hit = 5x normal dmg).
or the BB, probably drop the sea plane in favor of something else. A bit extra spotting is not really going to make much difference in capital vs capital battles. More guns, possibly secondary batteries if speed is a concern. Also, change that Secondary Batter II into a multipurpose one, they cost more, but 150 IC is not going to be much of a difference for a BB, and the extra AA is going to be very useful.
Again, AA those BB´s have is more than enough. Better focus on light or heavy attacks instead. The plane is for some individual action. Germany for example can use BB´s for hunting raids with other surface fleets making the normal allied sub-hunting partys take quite a beating.
For that the BB can´t be blind. But i agree allied BB´s can be virtually blind as they usually have other ships to do the job.
ASW: You want a Light Cruiser with nothing but sea planes, sonar and radars to actually manage to find enemy subs. DDs can kill them, but they don't have enough spotting to actually find any later subs. Depth charge capability is secondary, once a sub is found you don't need much damage to sink them, the issue is actually damaging them.
You are correct. I made a England guide where i did just that ^^. Where i had CL´s with seaplanes and a S&D party with depth charges. So one team to find and one to kill.
but for AI that is overkill.
This guide is not 100% MP oriented. Also i did found new ways to deal with naval problems after i completed the guide. In a week this guide will mostly outdated for MP gameplay.
But for SP its going to be valid and more than enough to beat the AI. (Unfortunaly).
This kind reminds my 1.2 naval guide that 1/3 was slight wrong or could be better, but still put people more or less in the right direction ^^.
Also for cruisers you seem to ignore that CLs are counted as screens, while CAs are counted as capitals, which is far more important than the penalties on deep/shallow oceans. The CL penalties for deep oceans are actually not really so, as they are to be compared with the other screen type ships, that is, DDs, which in turn have -50%, as you yourself have mentioned
by that point CL´s are actually even worse. A CA can be defended from torpedos and light attacks by hiding behind CL´s and DD´s. At same time they don´t get the penalty.
Also having a 50% advantage in damage and defense vs a Enemy CL (or even better a 50% vs DD´s) is huge and allow for a quick cleaning with little to no losses.
CA´s do have 3,5 worse visibility, that makes them a little easier to hit. (+11,6%). but the advantages compensate for that.
Further on cruisers, in particular light cruisers, their armor capabilities mean they can pretty much be immune to other screen's guns, as it's really hard to get piercing over 10 for your light guns. Best CL killer are probably CAs or other CLs, and they should cut a swathe among DDs.
Armor is a dillema. Armor I and II are pretty much more or less the same as its easy to reach 8 piercing with light guns.
Armor 10 i agree its useful and impossible to get with light guns alone. But also slow and expensive on resources.
DDs can't really find subs on their own, so they can't really do much killing. Using them as convoy protection is still useful, as they at least force the subs to flee and prevent damage, but the best subs for this are the obsolete hulls that can't really cut at actual battles anymore. Or even purposefully building earlier hulls for cost savings, if all you want is a platform to throw depth charges from.
I find a better use for 1918 DD´s to be minelaying/sweeping as 25Hp is too vulnerable even to submarines.
For Convoy Escort you already want the 1936 ones. Of course coutrys like US, UK and Japan have too many DD´s of 1918 and you can use them on safer shores while you build a modern screen fleet for your main strike force.
But you are correct

. Most the guide already being a little outdated the moment i launched it as i kept testing and improving ^^, still not going to edit as the hints are ok for SP .
As actual combatants, my current theory is that they are best used against enemies light on their own screens, as their torpedo capabilities are wonderful. For that purpose, outnumbering the enemy is extremely important, as they would need to go through the enemy screens ASAP. Still, for most of those, drop the dual purpose light gun, it costs double that of a light gun III, 150 IC extra per gun. They are fragile enough that I don't see the extra AA being that worth it, specially since it's not going to be a high number anyway.
The Double Purpose Gun is to give AA coverage for the whole fleet. 20% of the average is used for other ships, so if your fleet AA is too low your individual ships even with high AA will suffer.
You can drop to something like 3AA with just 1 gun, but having a good AA value is key to DD´s as they are the most numerous ships in a fleet allowing ships with low AA to reach a value of 5+.