One of my favorite places to eat fish and seafood is Boca del Rio, at Ribera de San Cosme #42, a stone's throw from Metro San Cosme. It's been around since 1941, although not at this location. You can read a little about its history on its website.
What I most like about it is that it's one of Mexico City's many time tunnels. It still feels like it's 1959 in there, with vinyl-topped tables, leatherette furniture and waitresses who are emphatic about their use of hair spray. The portraits in the lower left corner of the photo are of the original owners.
The coctél de callo de hacha (scallop cocktail) is also a big draw for me.
I almost always order the mojarra a la diabla, which is a whole fried perch in a chipotle sauce.