You need 75 tiles of OPEN water. Note that any water that touches the ceiling and all vertically beneath it is not counted - I tested with the fisherman's pocket guide. But note that you will have a penalty on your fishing power which reduces until the water is 300 tiles in total.
Note that the requirement does not calculate the exact amount of tiles filled with water - having 50 tiles wide of 1.5 tiles of water can be a valid fishing area.
On earlier phases where larger water bodies might not spawn near you base you can drain nearby pools of water to a open area under your base as an early game fishing area. Which you can later lower by digging it's base to make it a cavern/underground fishing area.