The best way to thicken seafood chowder (or any chowder for that matter) is to simply add some type of flour to the mix as I do in this recipe. If you still don't find the chowder thick enough, you can always make a cornstarch slurry by mixing equal parts of cornstarch and water then stirring it into the chowder. Alternatively, you can take it a step further and make a roux by mixing flour with a cooking fat such as butter or bacon grease which eliminates the need to thicken the chowder after the fact.