Do you mean fresh seafood or fish-based commercial foods? The problem with fresh seafood is the mercury levels (same caution should go to humans as well, but obviously it takes a higher concentration). The issue with fish-based commercial foods is the excess of magnesium and other minerals that are thought to cause kidney and urinary issues.
I feed my cats wet food twice a day to supplement free fed dry, which is usually a chicken flavor. I rotate the flavors between fish, turkey, chicken and beef, they get each 1/4 th of the time. I also feed one of the cats one shrimp when I happen to have it for dinner.. maybe once every couple weeks. Does anyone know if this is bad. We have no problems here with urinary problems.