Flies look for contrasts
I'm an entomologist that more-or-less specializes in mosquitoes and biting flies. Though frankly, when we're plagued by flies on the boat this expertise accounts for nothing. It happens to us in the middle of the Chesapeake. We can only resort to our MPT (Mechanical Pest Traumatizer).Many kinds of flies will be attracted to your boat in the middle of nowhere. The most common biting flies are stable flies (they look just like house flies but bite like hell) and various kinds of horse flies (greenheads, deer flies, strawberry flies, and others). All of them have lousy eyesight but are highly sensitive to contrasts and movement. This is probably why they flock to a boat far from land. Flies closer to land have plenty of other interesting things to be attracted to.The only recommendation I could make is to paint waves on your hull and dye your sails blue-gray. Yeah, that's the ticket.....EvenKeel