Forgot...the fuel thing...
Kayaks are really aerodynamic because they are built to be slick in the water. I've hauled two wide fishing kayaks (upside down) on my custom ladder rack on long trips to the beach and not noticed any decrease in fuell economy in my Titan. The boats basically ride in the spoiled draft coming off the grill and windshield. Main issue is stout cross winds. I can feel those sometimes.