A drainage basin is not defined by rivers, but by hydrology as a whole, meaning where runoff flows regardless of surface water features. Also, according to this map, even microcountries in and around the mediterranean with no river should be included in the Mediterranean drainage basin if the rain that falls on those countries eventually feeds into it.
I forgot San Marino, but according to datas I have found Monaco and Vatican do not have rivers.
A drainage basin is not defined by rivers, but by hydrology as a whole, meaning where runoff flows regardless of surface water features. Also, according to this map, even microcountries in and around the mediterranean with no river should be included in the Mediterranean drainage basin if the rain that falls on those countries eventually feeds into it.