how do I save this dish, I made a large recipe & it is expensive.
What else is in the casserole? I can think of several options that would all rely on rice to dilute out the salt and seafood flavor.
Depending on its texture, you could simply serve it over rice like a stew. A big squeeze of lemon will also help temper the perception of salt and almost always makes seafood taste better. Some chopped parsely might also help lighten and freshen the fishy flavor.
If it is simply seasoned or its seasonings are compatible, you could also jury-rig it into an étouffée (served over rice) or into paella. If it has a dairy component I wouldn't recommend either of these.
Can you dilute it and make it in to soup?