Accessible restaurants in Edinburgh
I can't personally recommend any fish/seafood restaurants in Edinburgh as I keep my seafood restaurant experiences to holiday trips to the Scottish Coast. However if it helps, when people think of seafood restaurant they often think of Fisher's - there's two one on the shore in Leith - one in the centre of town - the one in town has a ground floor entrance but I don't know if it has ground floor seating.
Or Sweet Melinda's which is next to the famous Eddie's Seafood Market in Marchmont and is a ground floor restaurant.
Hiya - the Tower Restaurant in the Museum of Scotland has lift access, and fantastic views across to the castle. It serves lots of local seafood - nothing massively adventurous and not cheap, but still, it's a great location.
Down at the docks, meanwhile, most restaurants I can think of are based on the ground floor - The Shore, The Kitchin, Martin Wishart?
hope that helps...