I can help a bit, but not completely.
1) There are no license qualifications to operate a recreational boat in the US. Some states do require boater safety certification, but as an "out of stater" I think all states will accept whatever is required in your home state. So I think if you can show you meet whatever requirements NZ has, you should be fine. And nobody is going to know what those requirements might be, so you should be fine.
2) You will need to register the boat somewhere. You could register it in a US State. See the state sales tax item next. Or you could register it in NZ and then take out a US cruising permit as a foreign flagged boat. Cruising permits are usually issued when a boat enters the country, so you might find our officials confused by a request for a foreign cruising permit for a boat that is already in the country. This probably warrants more research.
3) The is no federal sales tax. State tax varies by state and ranges from zero to as much as 10%. There are also events that trigger the tax being owed. The details will need to be verified for any concerned states, but in general you can expect:
a) If you are a resident of a state, then tax is triggered as soon as you bring the boat into the state. This shouldn't apply tp you since you are not a resident of any state (presumably).
b) If you keep the boat in a state for more than some pre-determined time, typically 60-90 days, the state treats you as though you are there to stay and tax is triggered. Some states have various cruising permits and other mechanisms to work around this.
c) If you are on a foreign cruising permit, you will not be subject to tax in any state. This would be an advantage to registering in NZ and taking out the cruising permit.
d) When you purchase the boat it would normally be subject to tax in the state where you buy it. But most also have provisions to waive the tax if i) you are not a resident of the state, and ii) you remove the boat from the state within some period of time that varies from state to state. You would want to understand this for any state where you might purchase the boat.
e) I think all states recognize sales tax previously paid to another state and won't double charge. But they often WILL charge for the difference if they have a highter tax rate than what your paid. So if you are subject to tax in a 5% state, then later become subject to tax in a 9% state, you be credited the 5% that you already paid, but would still owe the remaining 4%. But you probably won't run into this particular mud hole.
4) Advice? I'd figure out what it takes to register in NZ since presumably you will want to do that eventually anyway, then cruise the US on a cruise permit. Also, google loopy kiwi. There is a guy who bought a boat here. did part of the great loop, then shipped back to NZ. He researched it really well and seemed to have it figured out.
