Noxema Skin Cream .... the 'white stuff'.
I used to travel a lot in the high latitudes where such are ferocious. There are some quite stinky 'dopes' but the best I've ever found is to apply Noxema Skin Cream (the stuff in the blue jar) and lather it on. It also works for most other biting 'flies': green heads, midges, biting house flys, etc. The 'ferocity' of flies during the day dictates how 'thick' you put it on.
Certainly you want to have any CO2 source (engine, cook stove, etc.) off!!!!
Flies also avoid 'wood smoke' ... old fashioned 'punks' and the commercial 'mosquito' rings also work, .... even when underway - just light them and put them in your on-deck / stern-rail barbecue.