What about importing my watercraft from Canada into the US?

I read tons of stuff about importing cars and personal belongings into the states but what about Personal Watercraft? Everything I find involves going north instead of south.Here's a couple of my biggest questions about moving my toys down to South Carolina.

What documentation is required for crossing the border? I have all registration up to date and associated paperwork.

My one jetski has been modified for competition and no longer has the OEM serial numbers. Will this be a problem?

What documemtation do I need for the trailer?

If it's personal effects you just declare it at the border together with the rest of your stuff, there is no duty. You use CBP Form 3299 if it's unaccompanied, make sure you have a detailed list of everything you are importing.

As for the serial numbers, the only suggestion I can think of is to register it on an I-68 using whatever numbers are on it and show that to CBP.