As others have stated, air power is extremely potent as a naval deterrent. particularly in this rule set.
Fighters and bombers provide value for the entire period of naval-build-up. Japan get to use the air to wear down Russia the entire time. By contrast, ordinary naval units don’t do much but look scary up until the big battle.
Imho, there are 2 kinds of fleets: Fleets whose purpose is to protect trannies (protector fleets) and fleets whose purpose is to destroy other fleets (assault fleets).
Regarding protector fleets, the key “unit” is the loaded AC. The loaded AC provides great protection at a slightly higher cost than 4 DDs, but the loaded fighters can be used for other purposes too besides fleet protection while the DDs cannot even offshore.
Regarding assault fleets, the key unit WOULD have been the submarine. Super cheap, stealth and immune to air. But a submarine is useless in any other way, just like a DD. Fighters are slightly more expensive and slightly more inefficient than subs, but they can also be used for other purposes besides fleet assault, so the fighter is the key unit of an assault fleet while subs are good for early battle fodder. The only problem is that fighters need a landing spot, which dictates for either an AC for every 2 ftr (extremely expensive unit and totaly inefficient for attacking purposes) or for a strategic air base (like Western Europe or Norway).
The problem for the Allies is that the Axis has such a strategic airbase exactly where it needs it (Western Europe) in order to threaten allied protector fleets, while the Allies don t have an equivalent “Western Europe” in the Pacific in order to threated japanese shipping.