Viking, you are right, of course but fifty knots on the beam of a ferry is a mighty force and it is all about risk management for the harbour authority. Berthing is an exact science and you can’t take chances with 40-50,000 tons of steel when a few feet either way makes all the difference between success and disaster. It is the closure of ports that stops the ferries and, if they are at sea when it happens, they stay safely at sea until the port is opened again or go elsewhere. Happens regularly in severe weather.
Regarding ferries, they can cope perfectly well at sea in a storm. The problem is manoeuvring a shallow-draft vessel with high, flat sides at slow speed in a restricted area. Harbour entrances can also pose special difficulties, so it all depends upon the wind speed and direction in the harbour manoeuvring area and the sea conditions in the final approach and entrance. These can change very quickly ( with a major wind shift etc) so you just have to monitor their bulletins closely and hope for the best!