It appears that the latest version of the “breathalyser” law is now in place. It is now compulsory to carry (at least) two “single-user” breath test devices in the car with you, this applies to holidaymakers as well as residents. However, according to this article, it looks as if there will be no actual fine for not having one.
For a quiet life, I would suggest you get a couple.
Ours was €3 for a pair from Carrefour. I dare-say the ferry companies will charge a premium but hopefully not a massive one. I would guess but can’t comment that they will also start to appear in motorway service areas alongside power adapters and phone chargers.
Note also that they have a “blow before” date on the side. Not sure if the bobbies would notice you had an expired one, but… you were warned! 🙂