Well given that the Goodyear Marathon tires are the #1 most popular trailer tire and you very rarely hear of anyone having a problem with them here on the BBC, I wouldn't think that there's really a problem here.
I know that many definitely exceed 65 mph towing their boats without problems.

Trust me Bernie, put on car tires and you will have problems. I doubt that the trailer will track as straight, and the trailer will most likely sit lower because the sidewalls are not as stiff for car tires as they are for trailer tires. It's your money but I personally wouldn't put car tires on my boat trailer even if they were given to me free.