There is “luogo comune” (commonplace saying/knowledge) in Milan that holds “il miglior pesce non si mangia al mare, si mangia a Milano“. (You don’t eat the best fish at the seaside, but in Milan.)
Milan, of all places? In the Middle Ages, ideally located Milan became the center of commerce for northern Italy due to the vast network of canals which were dug that converged there from all over the Po River valley. So much so that together with Genoa, Venice, and Naples, at one point landlocked Milan was among the largest ports in Italy in terms of tons of cargo loaded and unloaded.
Due to this, one of the larger wholesale fish markets took root in Milan (still functioning to this day). Together with the city’s wealth (and appetite for fish), this mean that Milan has been considered the fish capital of Italy (…according to the Milanese, that is. Sicilians and Neapolitans might have a word to say about that.)
In order to maximize profit what certainly does happen (regional pride notwithstanding) is that many fishing boats all over Italy will divide some of their catch in two. Top quality fish and crustaceans get sent for top euro to the Milan market, while the rest goes to local markets. (Not exclusively, but quite a common practice.)
To get a taste of this top selection, head to Via Spadari. Pescheria Spadari is one of the best and most famous in Milan (most expensive too), but the selection is incredible. Also, the fish stalls at Porta Ticinese are well-known by the locals. Ristorante Pescheria Da Claudio on Via Cusani is another fish monger (now also fish restaurant) not to be missed.