"By the book," the safest way to avoid any foodborne illnesses is to take any frozen fish, meat, chicken etc out of the freezer and thaw it in the fridge. That said, if you're using it right away once thawed, placing it in a sink of cold water, or running water over it to thaw is a little quicker, but always make sure if doing that, once thawed it passes the smell and sight test. IF it smells off, or looks off, (or both) I wouldn't use it. Rule of thumb in the kitchen, "when in doubt, throw it out."