Not sure how well this works, but they make a spider net that you can hang over your slip area above your boat - Spider Net Boat Protection. We always just put our cover back on our pontoon at the end of the weekend -- and place dryer sheets around the boat (and sticking out of the under-seat storage compartments) to keep them off the boat when it's covered. When we do get spider poop stains on the seats, we use a Miracle Eraser to get them clean again. Also using 303 Protectant a few times a year helps get the spider poop off as well -- and it protects the seats from the sun damage.