I'm not sure I understand the question, but, if you have the work boat 'build' fishing boats, then there must be an aforementioned 'bonus' in the tile. If the fish (or crabs etc.) are in the net then the tile is being 'worked' by the city and if there is no net then this city doesn't yet have the population to 'work' it yet. I believe the health/trade bonus exists whether the works it or not after the fishing boats have been built.
When the Work Boat if in the same tile as the bonus the icons on the bottom of the screen will light up. Then you can 'spend' the work boat to become Fishing Boats. (later in the game ((Optics)) you can 'spend' the work boat to become Whaling Boats, and much later ((Plastics)), they can be used for Offshore Platforms for oil pumping)
Hope this helps