Mike,
4 or 5 years ago, we were staying in Nice, and used to use Cagnes-sur-mer beach - just along from you. The fishing was ordinary- mostly tiny bream like stuff which you could easily see if you went snorkelling. But one day, while watching my younger daughter reading a book on an inflatable about 20ft from the shore, there was a massive fish strike just behind her as a fish chased something right up to the pebbles (think perch but with a 3 foot high plume of water). I guessed it was a tuna or something similar - what sort of fish down there would do this?