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.