Eric.
I have been to Sorento twice, but the River there is very small, I did not see anybody fishing it, and did not see any fish, maybe the wrong river / location ?
I have fished the coast all around Sorento, about 2 miles south of the town, there is an old Roman Villa,not a historic site, accessed via the coast road, the area is volcanic rock, and very sharp underfoot, but is well worth the effort if you can put in the time, this area is very popular with local anglers, fish a largeish pike "Bung"with a flat top, about 3 feet up the line, 2 feet below run a drilled bullett stopped by a swivel, then attach a largish (2/0 +) streamer fly or similar, repeatedly cast this as far as you can and wind in rapidly, you will catch smallish Tuna up to about 6lbs or so, and what a fight they put up .The Pike bung acts as a splashing attractor to the Tuna, who snap at the streamer fly.If you choose to, you can fish smallish baits such as prawns, or fish strip,under a float off the rock marks, and you will catch a species very similar to Wrasse.
In the center of Sorrento there are several large harbours these hold Mullet and several other smallish fish, usual Mullett tactics will catch . There is a small marina called Marina Piccolo, which has a man made breakwater type of structure, locals night fish from this, using coarse tactics.. I have had Mullet from there using maggots purchased from the tackle shop in the town....there is a further "bay " to the left of Marina Piccolo, which I did not fish,if you do visit this bay be very careful, there are alot of used syringes underfoot, not very pleasant and surprising in such an area.
Capri has many rock marks that also produce as above.The harbour in Capri can be fished as those in Sorrento, again,Coarse tactics will produce many Mini species and the usual Mullet
The River Arno in Florence is full of Carp and big Cats, I have seen the locals fishing for the Cats right in the town Centre almost under the Ponte Vechio, in rowing boats. I have also seen the Carp shoaling to spawn just below the Wier near the island, nothing big, perhaps 12-15lbs, but lots of them.The local angling clubs run matches all along the Arno, but only seem to fish for silver fish, they do have Carp Matches during the summer though.
In Rome the Tiber is stuffed with Carp and believe it also has some large Cats, but have not seen anybody fishing for them.
I do not know what the licence requirements are for freshwater fishing, as I never tried it, but the sea is free for all.Local anlgers are very helpful.
Hope this helps a bit.
Dave.