But "ironclad" covers a wide variety of ship types - from the early stuff like the French coastal batteries, La Gloire/Warrior, and Monitor and Merrimack/Virginia - through all sorts of strange intermediate designs while people were still figuring out how to use the new technology - and then up to the pre-Dreadnoughts that were almost battleships.
That's true. But there were countless different types, classes, and rates of Age of Sail warships, but Civ5 only has 3: the Caravel, Frigate, and Ship of the Line. The Ironclad represents
all of the ironclad warships of the mid-to-late 19th century, which preceeded the all big gun Dreadnought battleships.
Just because the most famous Ironclads weren't designed for ocean travel doesn't mean that the Ironclad unit in Civ5 should have the same limitation. There were plenty of Ironclads that were ocean-going cruisers and battleships.
And here's the trump card that should end this debate: the Civ5 Ironclad's unit picture.
That picture is
not of a Monitor or Merrimack-style early Ironclad with a low freeboard. That's definitely an ocean-going ironclad -- it even has masts for SAILS!
Case closed.
I have to disagree on the demarcation of coastal vs offshore here - coastal tiles are still open water, and really the ocean tiles and their restriction is more an abstraction representing inability to make ocean crossings between continents.
I think there's two aspects to this - the first being the appropriate seaworthiness to be able to handle the worst of what the ocean has to offer without the ability to shelter in port or undergo proper repairs. A lot of the ironclad type ships sacrificed seaworthiness/reliability in exchange for combat ability - many coastal ironclads had ridiculously low freeboard or were ridiculously topheavy.
The second aspect is the inefficiency of the coal-powered engines, which puts a serious limitation on the operational range of some of these ships.
So while yes, there were plenty of ironclad ships capable of making ocean crossings, many of those were specifically designed around that ability - whereas given that most fleets were designed around fighting nearby European rivals, such an ability was not essential and dispensed with on many ships.
Again, you're focusing too much on the earliest and most famous Ironclads, and not the diverse assortment of ironclad warships of that era.
The fact is that the Ironclad's inability to enter ocean tiles is the primary reason why so few players build them. What is the point in building such an expensive warship if it's only capable of defending your coasts, and not bringing the fight to your enemies' shores.
The Ironclad needs to be modified to make it actually useful. If not, it should be removed from the game.
I'm not arguing that ocean-going ironclads didn't exist or even that they weren't common, but rather that there's sufficient historical justification for a rule that I think makes naval combat more interesting. But yes, they're way too weak to make up for it. I'd go for more combat strength to represent their toughness, and I'd also go with range 3 - which would let them play a more effective role as a coastal artillery battery, as well as slightly mitigating the coastal-only penalty by letting them hit more slightly-offshore ships.
The Ironclad's Combat Strength can't be increased because it's 35, which is the same as a Destroyer. Instead, Ironclads and
all Industrial and later era Naval units should be given a special promotion that gives them +50-100% damage resistance to ranged attacks from pre-Industrial era Naval units. Ironclads also should have higher Ranged Strength, because most were armed with large calibre rifled naval guns that fired explosive shells, whereas sail-powered Frigates and Ships of the Line usually only had smoothbore solid-shot cannons. The Ironclad's Range Strength should be increased to 20. The Ironclad's range should remain 2, because 3 is too far.
IMO the ranges of all pre-Industrial era Naval units she be reduced to 1, because even though they had ranged weapons, they weren't long range. I understand why Ranged land units have a minimum of 2 Range because they're so vulnerable to melee attacks. But unlike land units, Naval units can't be melee attacked, so reducing their range to only 1 should be fine.
/30
/5
. This combines with other changes to ships and their promotions to create more exciting naval warfare. 
The point is both types of ships should be present. They both had very different functions.
