AlexCaledin said:
So, how much better the monitors would have performed, coming much closer to the Germans and being almost hidden in the water and much heavier armoured.
Don't confuse the USS Monitor's extremely low profile as being typical of all monitors. Most were much larger. Here's a picture of the HMS Marshal Ney, a monitor-type warship commisioned in 1915:
That being said, 'modern' monitor-type vessels typically had very large guns for their size (15-inch warship guns you'd normally see only on a battleship mounted on a ship with a displacement of a cruiser or less) and would likely have made excellent shore bombardment platforms. Indeed, several were built during WW2 for this exact purpose:
https://en.wikipedia.org/wiki/Roberts-class_monitor
The HMS Roberts supported the landings for Operation Torch (Allied landing in North Africa), the invasion of Sicily, landings near Salerno, invasion of Normandy, and landing at Westkapelle. Her sister ship, the HMS Abercrombie, supported the Allied landings near Salerno.
That being said, they were just as vulnerable to counter-battery fire as any other ship was. The largest monitors (like the Roberts class) are much more heavily armored, but much slower than a destroyer, while the smaller monitors are just as unarmored as any destroyer and not nearly as speedy.
Here's a picture of the USS Wyoming, the last U.S. monitor-type warship:
And the HMS Abercrombie:
As you can see, these are a far cry from the original USS Monitor, bearing a design that is much more in common with contemporary warships of the time than of the Monitor.