You should try Riddle & Finn, either in the Lanes or on the seafront. A bit expensive but really great Fruits De Mer.... start with a plate of Whitstable oysters washed down with a frothing glass of Guinness.
I'm a Brightonian who loves jellied eels. When I worked in London, there was a plaice near London Bridge where there was a stall run by Tubby Issacs. There's nothing better than standing up to have a plate of eels smothered in malt vinegar and white pepper with a bloody great hunk of...