You can cook anything in a crookpot....I would think the most common recepies for fish in a crock pot would have to be soupd stews or curry's...
Here is one I bookemarked a while back
WhiteFish Au Gratin Recipe
3 pounds white fish fillets
6 tablespoons Butter
3 tablespoons Flour
1 1/2 teaspoon Salt
1/2 tablespoon Dry mustard
1/4 tablespoon Nutmeg
1 1/4 cup Milk
1 1/2 teaspoon Lemon juice
1 cup Cheddar cheese -- shredded
Melt butter in saucepan on medium heat. Stir in flour, salt, mustard and nutmeg. Gradually add milk, stirring constantly until thickened. Add lemon juice and cheese. Stir until cheese is melted.
Place fish into crockpot, spooning sauce over fish to cover. Cover and cook on high 1 to 1-1/2 hours or until fish flakes.