Great find! Based on the picture and location, I believe this is the **Corsican grass snake** ( _Natrix natrix corsa_ ). It is a pretty rare subspecies, and is listed on Annex IV of the European Union Habitat and Species Directive. Worth mentioning that it's nonvenomous.
![Natrix natrix corsa](
Photo cred to Frank Deschandol/Philippe Sabine
Unfortunately, I was not able to find too much more info on this subspecies. I know _Natrix natrix_ are pretty widespread throughout Eurasia, but this subspecies is much more restricted.