There are two big supermarkets in Barrio China that seem to have fresh seafood (at least fresh enough for me). I don't remember their names, but one is on the 2200 or 2300 block of Arribeños (river side of the street), and the other is on the 1600 block of Olazabal (on the side of the street towards Nuñez). Both are pretty identifiable as supermarkets; just go in and go to the very back of each, where the fish are.
There's also a little fish market in Palermo on the corner of Santa Fe and Jeronimo Salguero or Julian Alvarez. It's very small, but right on the northeast corner and very obviously a fish market. A friend of mine who knows food thinks it's excellent, although I haven't tried it yet.
