@NewGuy
I bought my first boat out of Tennessee, brought it into Kentucky. I had to file for titles with the local county clerks office. To do this, I had to take both the boat and trailer to the clerks office (There is one in particular in Louisville that does it), and have a sheriff do an "inspection". Basically they just check the Hull ID against the certificate of origin, or the bill of sale. Once they have that, they will issue a new title in the state it's being registered in. It was my responsibility to list the new lienholder on the title. The bank was less than pleased with this process, but I was able to talk my loan officer into the idea that I was a trustworthy individual.
Also, my trailer was built, then sold, then used in TN exclusively before I got it. We couldn't find a VIN anywhere on the trailer, so I had to have a new one issued, then had to stamp it on with punches, then have it inspected and registered. It was a humongonomous pain in the arse. I doubt you'll have this issue with your trailer if it's under a Yamaha boat, but be aware it could happen.
Finally, KY doesn't require registration on the trailer unless you leave the state, so I never had a plate on it. I sold it to a woman out of Indianapolis, and she had to jump through similar hoops to get it registered and ended up making a trip BACK down to Louisville to get me to sign some additional papers to make it happen.
Overall, there has to be a lot of trust in the people working deals around this kind of BS. Because at some point you'll find you just don't have the thing you need to get the process complete and will have to go back to the other party and get something signed, or get some additional piece of paper. I've imported a car from Canada into Indiana with less PIA than I have transferring between states.
Good luck dude!