I have just bought my second boat in the US. No problem.
You cannot register an American Vessel unless you are a US citizen or Resident Alien(area 51)
You can register the vessel in the state and transfere title.
You cannot insure vessel in US unless you have a US mailing address.
A non resident Alien can insure a vessel which is kept in the US and use the location as a mailing address.
If you do not keep your vessel in US. You can fudge address until your claim is denied. #rd party would still be covered.
You can insure a US state registered vessel outside US. I have to pay a one time charge for cover until I leave US with boat.
Depending upon state you May or may not be required to pay sales tax. if you are required to pay you may have a time limit to take vessel out of state and pay no state tax.
Rules for importing into NZ would be up to NZ.
I imported into Canada
. declare at time of first entry and pay up. registration made no differance.
No expert just the problems and solutions I have run into.
Crossing back into US on a US state registered Vessel as non resident and Citizen of an other country leads to lots of questions.
Like why do you own an American Vessel.
Why have you not changed the registry.
Dont say I just haven't bothered yet.
That leads to more questions.
Changing the name and registry is bad luck. went over much better. even though he thought I was nuts.