Okay I will give a complete view on the current Sea balance.
Current roles of ships:
Submarine: By far the strongest in Attack. Even the strongest in Defense, when paired with full carriers (5 subs for each full carrier) due to being cheap cannon fodder. Strong convoy. Requires opponent to have enough destroyers to kill all subs if you decide to spread them for maximum convoy. At the moment the strongest ship and actually too powerful.
Destroyers: Counter submarine abilities. Cannon-fodder for air-only attacks. Blocker unit.
Carrier (with planes): Provides a longer attack range, giving you high versatility. Planes are not prone to being blocked. Supports Amphibious Assaults. Planes can be used on land, when they are not needed on the sea anymore. Soaks a hit per carrier.
Battleships: Bombardment. Soak a hit per BB
Now with a little math, that don’t need to be shown here once more, it is obvious that Battleships are inferior to destroyers if you only look at damage in offense and defense and hp. Since the destroyers additionally have other uses (blocking, countering subs, cannon fodder for your carriers), battleships are simply too weak, period.
Cruisers are even far weaker than battleships and don’t provide more abilities than battleships. If the battleship is already too weak so that it’s rarely purchased, that means cruisers are just a joke.
Now first of all bombardment needs some work. Carriers give you far more power in amphibious assaults alone, and that’s not even counting the numerous other advantages carriers have compared to bombardment.
No matter how bombardment is changed: I am convinced that the cruiser < - > battleship balance will never be solved, if they share the same ability (even if the bombardment differs slightly).
As in every balanced strategy game (doesn’t matter if it is something like chess, or something like StarCraft), every unit needs its specific niche role.
Whenever, in ANY strategy game, two units share the same abilities, math will always find the better one. If they are both equally good, the more expensive one is obsolete.
Therefore I’m not in favor of cruisers and battleships sharing bombardment (and additionally bombardment in its current state is too weak to justify buying either of those two ships).
A possible solution
Reduce the cruiser cost to 10 and let them bombard every round. This would lead to cruisers being equally good to destroyers. While destroyers have their uses described above, cruisers would be the best unit for amphibious assaults. They can’t be shot down by AAA like planes, so cruisers would finally become viable.
Reduce the battleship cost to 18 and remove the bombardment ability. Instead give them something different. A possibility would be a rocket-like attack (like the technology rockets). It kinda makes sense, that huge battleships have the skill to damage facilities. One non-blockable 1d6 attack on a coastal harbor, airfield or factory per bb and territory.
This would bring the battleship in line with the 10IPC cruisers IPC-wise. They are slightly stronger than destroyers. I don’t think that’s a big problem, since at the moment destroyers are having a lot of uses already. You’d still buy destroyers and they fill in the role of cannon fodder even more (besides, you can’t always spend 18 IPC for one ship anyway). Battleships on the other hand would be the go-to unit for besieging, in addition to convoying subs.
Increase the cost of submarines to 8 and increase attack from 2 to 3. This will lead to submarines no longer being used as cannon fodder. Submarines would be more like bombers - attack-units only. And it makes way more sense historically.