I will agree with the weight being about the same or close, but trailer tires do take a little different abuse then car tires do. Trailer tires are 6 ply(cars are 4 ply) which also means they have a much stronger sidewall, beings they also don't have any turning components/suspension like a car does. This means the trailer tires, especially a tandem axle, take alot of abuse to the sidewalls. If you don't believe me, just watch a tire on a trailer in a tight hotel parking lot and see how much a trailer tire takes as far as load on the sidewall. Also the suspension on a boat trailer is way less forgiving then what's on a car. Basically the tires on a boat trailer is the suspension--sort of speak.
Car tires will work for the most part----I just think ture trailer tires will last longer,are stronger and can take more abuse. Cost----not a great deal more for a Marathon vs a good car tire of same size. You can buy cheapie 205/70/14 car tire for about $50, but that's exactly what you are getting. Now a better tire might run in the $75-$100 range and a Marathon runs about $100. Not saving alot.