-About the navy it's use is to bombard coastal cities and land units the crusier and BB can do both while we need two different bombers, I think the navy is more rounded for city and unit bombardment and ship combat while the airforce is more specialize, also the NB is effective againts ships your right there, but the navy is more practical they can do three different things and the airforce can only do one at a time but way better. Of course the basis of this argument is IF you have access to the water or need the navy such as US, Canada, and maybe even UK and all the Mediterranean based countries like Italy, not all countries will be relying on their navy but to a certain few the navy prevents them from being completely surrounded (Italy).-
2 units in the navy : Cruisers and BB can do the 3 roles (ship , unit and city combat) but for their high cost do they do well? lets have a closer look:
The cost of a BB is twice the cost of each : NB, SB and TB. This means that we could have invested in 2 bombers instead of the fancy BB.
What are you planning to use the BB for?
Attack Ships? - Get an NB instead and you can still afford yet one more bomber
Attack City? Get an SB ;
Attack Land units? Surprise ! You don' t need a TB , did you know that an SB deals almost the same damage to infantry and tanks as a BB!!
So with the amount you investedin a BB ,you could be having an NB as well as an SB that are doing the job much better as you can bomb the navy and the city at the same time where as using the BB you will shoot only one target at a time.
As for Cruiser: they cost 30% more than each of : SB, NB and TB and cost twice as the Interceptor.
With the cost of a cruiser , you could instead have an NB and an Interceptor, Dont laugh yet , Did you know that an interceptor deals the same amount of damage against tanks and infantry as a cruiser?
Plus Cruisers dont have impressive stats against cities, nothing that would justify their cost.
-----
As you can see , I have did a very thorough study and the current Navy ,namely the DCB (Dest,Cruise and BB) are not cost effective. Iam not saying that you should not use the navy, but there is an obvious balancing problem here.
The DCB needs a buff , granted, but altering the stats of so many units would be complex and might cause unwanted side effects and issues,for which i suggest adding a mechanism that increases the importance of a navy. Afterall , one of the main purpose of navies now and back in WW2 was to protect merchant ships that bring in supplies to the country.
This would be easy to implement , one button to enact a Naval Blockade when you have a DCB fleet near the shores of a country and an error message that tells you : You cannot order supplies from the market due to an enacted Blockade.
Ofcourse , the country can still trade via private trade (messages) and so it is not unfair, it adds a strategic dynamic and showcases the true purpose of navies. Navies shouldnot be better than an airforce in destroying targets but they are missing a crucial economical damage potential : A Naval Blockade.