To Make Ahead: Package fish fillets and freeze until ready to serve. Store new potatoes until ready to serve. Whisk remaining ingredients in a small bowl to make marinade. Freeze in a plastic bag along with fish fillets.
To Serve: Thaw marinade and fish fillets. Marinate fish 30 minutes. Reove from marinade. Broil or grill fish for 10 minutes, basting frequently with the marinade. (If fish is more than one inch thick, turn once during cooking.) At the same time, quarter new potatoes and toss with Thai fish sauce. Bake with fish.