All seafood should be precooked before adding it to the chowder at the end. The exception to this rule are the shrimp and scallops. You can add these in at the end immediately after you bring the chowder to a boil and then decrease the heat. Let the raw seafood cook for 5 minutes. The shrimp and scallops cook very quickly, make sure you don’t over cook them, the may have a chewy texture if you do. See the illustration below.