Why USS Monitor's low freeboard made ocean passages dangerous

How serious was the freeboard problem on USS Monitor?

It was fundamental: monitors were not rough-water ships, and Monitor's loss in the Atlantic demonstrated the danger of taking such a low hull into an ocean sea.

A monitor's fighting deck sat so close to the water that waves could sweep the ship and overwhelm openings or pumps during an ocean passage. USS Monitor's sinking off Cape Hatteras is the clearest consequence of that design tradeoff. When Cerberus had to travel to Australia, builders added temporary structures that effectively increased her freeboard for the voyage. The comparison shows that a coast-defense hull could be transported across oceans only by acknowledging that it was not designed to fight the sea itself.