MARK !! I thought you know the sea !!
Look at magicseaweed and see the differences between high and low tide.
Sometimes its more than 5m and sometimes as little as 2.
The tidal differences behave like a sinus wave. Around slack high and slack low, you have very little flow and mid tide its the most.
As a consequence, the boat might drift during one 10hr fishing trip first 3 m/s to the north, then approaching slack low and almost standing, only to start drifting southwards.
To make matters more complicated, the tides also depend on the line-up of sun-moon and earth. That is why you get stronger and weaker tides throughout the year and not only undulating within the lunar month.
Have a look at Shy Torques webpage. He indicates, when a tide is good or bad.
Another aspect is the depth of the water. The deeper the water is, the less does the tidal flow influence the fishing. That is why some skippers go well offshore at the big tides and catch very well.