When I decided to cut back on red meat, I started eating a lot of baked fish, mostly salmon filets and tilapia. I can see where eating nothing but fish as a protein source at every meal would get old, but I don't know that it would be unhealthy. I think when you talking about unhealthy fish, it's mostly those deep ocean fish like swordfish and tuna that pick up all of those poisons. Farm-raised catfish or rainbow trout probably doesn't have all of those contaminants in them.
I will say that fresh fish can be a lot like raw chicken. You had better handle it right and cook it before it has a chance to get funky. The problem with sushi is that if it stays out on a buffet bar too long, it can make you sick. A typical fried fish sandwich with tartar sauce from a fast food place may not be any healthier than a hamburger, either.