looks very much like Cyathura carinata we have here (Netherlands), but not sure if there are siblings in Italy. I have added a photo of a Dutch specimen
It is an Anthuroidea species for sure. The estuarine character of your sampling location indicated the possibiloty of Cyathura carinata, but it can also be another species.
Please check Wägele (1981) Study of the Anthuridae (Crustacea: Isopoda: Anthuridae) from the Mediterranean and the Red Sea. Israel Journal of Zoology 30: 113-159
With a better picture of the telson, I think we can be 99% sure of the species identity. In the Scheldt estuary, we also have C. carinata and it also dwells in the oligohaline and freshwater reaches of the river.