Vienna's Kutschkermarkt is an interesting place. Nestled into a steep side-road in the rather upscale 18th district, it has a few delicatessen places that sell good cheese and some Italian style sausages, but in general it's not unlike most other markets in Vienna, only far pricier. They do know their audience, after all.
One thing that sets it apart from many others, though, is a place called "Atlantis Fisch". Camouflaged as an ordinary fish-monger, it doubles as a restaurant and if the weather allows it, small tables are put up in front of it. They then serve you the fish you can see on display, prepared in the tiny kitchen of their market stall.
The owner is a nice fellow, and his staff is always equally forthcoming. And their seafood platter is to die for. Not just for the fish that are served, obviously. The image above is of one of those platters. As you can see, they include a whole fish, fish filets, langoustines and some delicately grilled squid.
I've been there a few times with my girlfriend, who is quite fond of their mussles in white wine, as well.
They're not terribly pricey, but you'll pay the price of fresh fish prepared well.
Here's their tiny website.