That's true, but when things get cheaper you can afford to lose a lot of them. Suddenly every container vessel is suspect. That trick has a lot of potential and harbors are relatively soft targets and easily accessible from just outside international waters. You could do a shitload of damage to most countries by just targeting a few key locations well within the reach of a basic drone and what sub $1000 drones can do is changing by the day.

Armor and artillery are basically useless against a fleet of seaborne drones.

Fair, it's not an aircraft carrier. But you can turn any container vessel into a cheap rough equivalent. Take the coastline, then maybe 30 km inland and see what installations you could reach. Pearl Harbor on a shoestring budget is a realistic threat now.