In the case I have in mind (people trying to get from Lybia to Italy), there's no country to speak of in Lybia currently. Even if there were and it wouldn't agree to the EU ship entering their waters, you can always get very near to the water border, inflate another raft for the people and board them on it. They'd have to sail to Lybia by themselves.

The bigger picture is that these situations would very quickly stop happening if the word spread that it's not longer viable as a way to get into the EU.

I wonder if returning them to Libya is actually better than letting them drown.