Depends on who you ask. I ate all kinds of seafood (especially tuna!) when I was pregnant with mine. It wasn't til the last baby that I heard no shellfish or canned fish more than 2 times a week. I can't say that I held to that rule very well, but I didn't go wholehog and eat it every single day either. Ask your doc what he/she suggests and try to follow that rule.
You can't eat anything that lives in a shell: lobster, crab and I think that includes shrimp. Nothing out of a can either - no tuna, mackerel or swordfish. All the rest is fine - catfish, tilapia...just nothing out of a shell and no rare fish.