Frankly, Erusea is only lucky that they really didn't kill any innocent civilians smuggling military hardware into a country, ESPECIALLY a country you're still officially neutral with, is a full-pelt war crime (unless the cargo ships suddenly raised Erusean flags to declare their allegiance).
As for how such a small attack can cripple the Osean Navy so easily, it's simple: They attacked the fuel storage facilities with missiles/rockets/bombs, and than had the drones slam into ships or drydocks.
Also, the drones attack more than just 4 ports, they attack what looks like 7 in the news broadcast, and then attack multiple staging areas at the same time.