TFB, there were many trips in Inland Saltwater through the years that left us in similar situations. The best Fish-vice I can give You...set out the Cut'n Board, Knife, Hose, Large Bowl or Pot. Get a heavy dose of Water In You, and make the 1st Cut on the 1st Fish. Keep Cutting. Know, that every cut You make, is 1 cut closer to Done. Try to not anticipate Every Next Whole Fish with Dread, but with Thankfulness instead. Don't get in a rush, trying to get done, but take Your Time so the fillets come out their best...as the Pretty Fillets will motivate You..."butchered" fillets will discourage You.
Don't let Yourself get distracted, this can cause You to "mess up" or even cut Yourself...stay focused on the Job. You'll be done before You know it...and with a Serious Mess'a Fillets.