I fish on the Texas Gulf Coast. I need a great knife to fillet fish...usually the fish are 15 to 25 inch specked trout, if that means anything to you...in other words, saltwater fish but not giant or deep sea size.
Are you processing the fish on the boat or when you get home? Do you know how to sharpen? Are you right handed? How much did you want to spend? Were you looking for a Japanese knife or western type knife?
Traditionally Debas are used for filleting fish. If you wanted something really nice super sharp you could try this: http://www.chefknivestogo.com/kabl2de21.html There is a bit of a learning curve to using debas but they work well.
150mm petty may work for those size fish. A 6" boning knife or Honesuki may also work, too (I've used both on cod before). I believe Mark has a 6" boning and a 8 or 9" fillet knife in the works for the Artifex series as well.
I have my 180 Tojiro Deba which I didn't get to use as much as I would've liked last fishing season, but I did break down a few smaller Bluefin and loved it. I would like to try the Masahiro for bigger fish......hell, any fish.