Yes. Heavier things require more power and reduce mileage. Does your boat weigh more or less than 0 lbs ? If you attach literally anything to your vehicle, miles per gallon decreases. Even a grocery bag
My best was 17 mpg without boAt, get about 12-13 with boat. Both my boats are lightweight aluminum, one of them way lighter than the other
we had friends take two rooftop kayaks to the beach, mpg dropped from 33 to 19. From 2 kayaks on J racks!