everything is based on the mandated watch structure of the vessel. As far as I know there are no uninspected fishing vessels under 300grt that have any watch structure requiring licensed personnel so if you don't have any licenses on the vessel and no legal watch rotation then I believe all you can get is what is allowed in the Small Vessel Sea Service rules.
If your fishing vessel was over 300grt and had a licensed master and mate, then as a two watch vessel you would be able to get the 1 1/2 day seatime credit.
I believe that all uninspected vessels only require a master and mate except for towing vessels on voyages of over 600miles which also require a 2nd mate. I know they did change the manning rules for fish processing and catcher vessels over a certain tonnage and number of personnel aboard to carry ABs but to the best of my knowledge all still only require two licensed deck officers.