When I use the Tide Forecast option, the F & up arrow symbols appear on the map overlay.
What do these mean?
These arrows show the movement of water level due to the tide. In the map here above, the level is decreasing (downward arrows), corresponding to ebb tide and when the level is increasing (upward arrows) it shows flood tide. If you click these small pictograms, you get more details about levels and tide hours. More explanations here: