In my many years of playing this game, I have yet to ever take naval ideas outside of a bad ideas guy run. With the exception of tall, isolationist island nations like Britain or Japan, all disputes are settled on land.
Even if that were the case, it is much safer to take quality ideas in order to mitigate an otherwise potentially insurmountable disadvantage to any armies that do land ashore. Quality also provides naval bonuses, in addition to far superior social policies.
Plutocratic, Aristocratic, Quality and Defensive all contribute to something beyond land warfare. Offensive and quantity contribute
exclusively to land warfare, and are considered the best idea groups in the game. Naval contributes
nothing to land warfare, and is considered the worst in the game. Do you see where I am going with this?
In one of darkfireslide's latest videos, he suggests adding something as insignificant as a +2.5% discipline bonus, under the pretense of superb sailor training increasing the efficiency of land armies by osmosis. But then you could make an argument for Switzerland take naval ideas, and magically increase their discipline with 0 sailors in the bank.
How do you increase the effectiveness of land armies, but strictly through cooperation with the navy? The central point, and the proposed solutions, are as follows:
Sieging coastal forts
Currently, coastal forts are strictly superior. At best they are -2, at worst they are equal at +0. Historically, we know that naval bombardment could make coastal forts more vulnerable, rather than equal to the always inferior inland ones.
Once the specific naval idea is unlocked I would suggest allowing coastal fort provinces to be blockaded up to 150%. At 150% you'd get +1 siege progress. Add a naval+XYZ social policy, that would give further +1 siege from 200% blockade.
Naval bombardment of forts
-33% cost of artillery barrage for 100% blockaded coastal forts. -50% cost for 150% blockaded coastal forts.
Heavy ships count as artillery
Another option could be to have heavy ships count towards the artillery counter when sieging coastal forts (yes I know it's unrealistic for the time period). Perhaps cap it, so that 50% of the bonus must come from land artillery. Big ships are much more expensive than artillery, but with this change they wouldn't be mothballed as much.
Amphibious landings
How about -1 instead of a -2 penalty for crossing straits or disembarking from transports with naval ideas. Additionally, how about +33% movement speed to, and from, transport ships. How about faster strait crossing?
Coastal batteries
Ships blockading coastal forts of a player with naval ideas could take attrition from coastal batteries. Say, 5% attrition at 1% blockade, and 0% attrition at 100% blockade. If 150% blockade were to be available to everyone without naval ideas, tweak the values accordingly.
This way, players who neglect their navy will find their coastal forts being bullied by players who don't. I've also considered increased devastation in 150%+ blockaded provinces, but the attacker might find that counterproductive.
With this flavour of bonuses, I could actually see myself taking naval ideas in WC runs, where praise of the +20% offensive siege ability is currently sung. Better coastal sieges and/or amphibious landings may be enough to sway people into, God forbid, actually taking naval ideas over quantity. Even if players were to take naval ideas strictly in order to conquer the micromanagement hellholes that are Indonesia, the Carribean, etc, I'd still call it a victory.
Adding all of the above bonuses into the game would probably be overpowered, so bear in mind that I pulled the numbers out of my posterior. Tweaks and playtesting are necessary, maybe nerf the values and sprinkle beefier ones around in the form of social policies. This could require maritime ideas to offer blockade resistance. Maybe everyone should get the ability to blockade 150%, and change the default siege penalty to -3?
On a side note, how about some flexibility to make sailors more desirable? Such as a "draft sailors" button, that could change, say, 2 years worth of sailors into manpower, but cost -2.5% discipline for 2 years each time the button is pressed? And vice versa for manpower into sailors.
What do you guys think?