You want a firm white fish that won't fall apart in the stew. I almost always have barramundi fillets (a sustainable choice) in my freezer, and I currently have sheepshead that my husband and I caught back in late January. Snapper, cod, halibut are good options. Shrimp is so widely available, but know your source. If it's not wild caught or "responsibly farm-raised," you may be disappointed. I cannot think of a seafood option that wouldn't be delicious - clams, mussels, crab, oysters.