This tip may be one of the easiest but will save a lot of aggravation. Make your boat a  " No fly zone " by keeping a can of "Off" or like product always on board. I spray all lower areas of the deck, as well as my arms & ankles at the first sight of a fly. Usually on hot days with little or no wind. So on those days when flies are biting more then the fish , you can concentrate on the fish. Good luck.