Boat covers confuse me. At 5,000 UV light is hard on them. Wal-Mart covers last about 2-3 years. I bought a fancy fitted cover from Cabela's for $285. All the attachments rotted off within 2 years on a cover designed for a 7 year guarantee. Currently I have a large silver 10 mil tarp on my boat that cost about $40. It is working fine.
I never tow with a cover because the wind wants to tear them apart. I have enough trouble keeping the cover on when the boat is parked.