We used to buy fish at the Marigot market, and there were always guys to scale and fillet the fish for a dollar or 2, depending upon how many fish we bought. Fun to watch them using a heineken cap nailed to a handle to do the scaling.
In April, I noticed a fish market in one of the stalls, maybe across from Le Sucriere, that appeared to be open most days in the morning that had filleted fish for sale.