So for clarification. The boat's owner is responsible for paying tax on a boat in their waters. So what is the tax?
In States that have these taxes, you'll owe a sales tax, if the sale took place there and you didn't remove it from that State quickly, or a use tax, if you bring the boat to the State and use it there for more than a defined period of time. 90 days is typical.
Let's say the NEW boat was $100k and the state where the transaction took place had a 5% tax... but the owner moved the boat to a where there was a 10% tax. If I understand you Owner had to pay $10K to the state where the boat is moored.
If an owner paid one state 5% as sales tax and moves the boat to another much later, this is a matter of reciprocity. The second State is now trying to collect use tax, as the asset is being used there, the sale did not take place there. Most States have a reciprocal policy whereby they will credit use tax owed, by whatever was paid in Sales tax elsewhere.
Use Tax is typically owed on the value of the asset, while it is being used in the next State. Not on the sale or the sale value. Sometimes they'd have a rule that said the value is presumed the same as the sales price, if it was less than say 6 months ago. However, if you relocate a boat 10 years later and it's worth half what you paid, you are taxed on it's current value.
But how is this actually enforced? I suppose to register you need to show proof you paid sales tax.
The attempts are varied, but many high taxed North Eastern States, who are coincidentally in deficit fiscal situations, are getting more aggressive with this. I understand that Mass marinas are actually required to report any vessel that is transient for more than 2 or 3 weeks.
Taxing authorities will also make assumptions, based on the mailing address one uses on State registration forms or USCG documentation. Note that the rules requiring registration or documentation are not the same as taxing. They get confused by some states collecting them together. Registration is a use permit, not a use tax, if that helps clarify. Nevertheless, the States will see a mailing address in their boarders and try to assume the boat is there. One needs to keep all documentation to prove otherwise.
The mailing address on my USCG documentation is not RI (I have more than one mailing address), even though the boat has always been moored in RI. I received a silly use tax bill, for maybe double what it would have been, if I had really owed it. I had to show all my marina slip bills, from the date of purchase to prove it was never in my mailing address' state. Once I sent them, they called to thank me and say the file was closed.
How is all this practically enforced?
In general terms, just like all taxes are enforced. There is a bit of random auditing and few systems to try to trip you up. Not everyone is caught cheating, however, if one was caught, the penalties and interest are painful.
No problem legitimately avoiding owing taxes, but that means not being in place where they are due. Otherwise, one should pay their tab. Cruisers can be tripped up by staying too long in States that enforce use taxes. Not hard to do, if you lay up for a season, for example.
RI doesn't charge sales tax for a reason. It draws in 15,000 boaters who are there spending money in their state, paying sales tax on all sorts of other purchases and keeping their residents employed, in marine businesses, so they can pay income taxes.