Well, I think someone has already mentioned this -- but one obvious difference is that the drones are not launching missiles at commercially important areas.
Once you start sinking ships without warning or cause, it starts to chill commerce on the high seas. And that's a very different effect from anything that happens from a drone.
I would also say that most of our international maritime law -- and a hell of a lot of domestic maritime law as well -- has been structured around the idea of piracy. And IIRC (I did a lot of research on piracy for a case a long time ago but I don't remember the details) piracy was the first action declared to be contrary to the "Law Of Nations." That is, international law only exists because of piracy on the high seas.
So for the US to engage in piracy -- which is what this is, piracy not actually requiring any seizure of the vessel; sinking it is also piracy -- would be a big deal.