If you have to be thrifty, then sell the boat and find another hobby. The last thrifty boater I saw on the interstate had a catastrophy and it damaged two other cars traveling beside him. By the grace of God MY family didn't get involved. You can be sure if his flying debris had hit my car, my lawyer would have taken his firstborn.
I'm happy that this bad idea got so much negative response.
ever hear the term 'assumed risk?' There is a risk inherent in almost every activity you do in daily life, and you just have to assume it is there. The typical example is getting hit by a golf ball on a golf course. Generally, you aren't going to be able to sue for that.
In this case, show me the law that requires a non-commercial vehicle to use trailer tires on a trailer. (hint, you won't find one, it isn't illegal) You assume an inherent risk by driving on the road that at some point, something might hit you. It might be a rock, it might be a semi tire that tossed its tread, or it could be a blowout from a passenger vehicle pulling a boat. Your lawsuit will end up in the same place in all cases, nowhere.
In general, I'm always amazed at the faith that iboats puts into very poorly made chinese trailer tires. A properly rated LT or even passenger car tire can be a very good trailer tire. And yes, I know, 'stiffer' sidewall. Don't buy that one either, as the steer tires on a passenger vehicle put on hundreds of times more force against the sidewall. The tire sidewall doesn't cause sway, improper loading does. I generally run trailer tires because of cost, but I have run non trailer tires in the past, and they all generally outlasted their trailer tire rated brothers.