Raw Bar in South Beach
Don't think you're going to have much luck finding that kind of seafood place on South Beach. There are several places that do raw bar (Nemo is probably the best, but definitely a "fancy seafood entree" place) but none I can think of like what you're describing (i.e., Tark's).
You may want to try Le Bon on Lincoln Road - Belgian, with a focus on mussels but I believe they also have a raw bar too. Great deal 5:30 - 7:00 you can get a 1/2 kg pot of mussels and fries for under $10.
Haven't been in ages, but I bet A Fish Called Avalon on Ocean Drive also does a decent raw bar. Again, more of a "fancy seafood entree" place though.