I'm not sure these are the same creatures, but we used to encounter big biting flies when doing the ICW. I distinctly remember them between Beaufort SC and Thunderbolt GA and many spots I don't remember as distinctly. They seemed to be very territorial, and on the fast sport fishing boat deliveries (25 to 35 knots) they were usually only a bother for 20 minutes or so. I couldn't imagine passing through these areas on a slower boat, taking hours or most of a day, unless you had inside steering.