If youre throwing that much debris up on your boat when towing, I'd suggest a set of big mudflaps for your tow vehicle. I usually put an extra thick coating of wax on the hull when Im going on a boating trip. Local day runs dont get bad.
I'm thinking of making a bra for the front of the boat- road crap is inevitable. You can also buy an adaptor that goes onto your trailer hitch- it has a long mud-flap looking thing to block any kick-up from your tow vehicle. Ask about them at any good RV sales place.