First of all you need a mix in fleet and in land units. Second, even at 20 a BB would be a sitting duck for 18 IPC of subs. 3 subs CRUSH a battleship with an average loss of a single sub costing 6 IPC. If all you had were BBs I would flood the ocean with subs and pick and choose when i want to fight. So you would need some DDs but still subs would kill you, so you need air units combined with DDs to project power. In order to get in striking distance of subs, because you have to move up you have to put yourself in harms way, let the enemy decide to attack you or not. You might need blocking DDs to get your fleet close enough to project power onto the subs and force a retreat as they can't stand a few DDs with airpower. In other words, you need combined arms to win. That is one reason I hate to use a single bomber against an AA. Going from 1 to zero is a 100% loss of the service of a bomber and the power it projects over a wide swath of area. Think about it, a single bomber might cause you to defend against it in 6 different places. Even by doing nothing, the power it projects influences the battlespace.
The subs cost 8 IPC in revised
7 if you posses ˝War Economy˝ NA