All states are different - California is expensive
California is the worst on taxes, I am now having to pay 9.25% on the purchase price of the boat i bought last year. Sales tax becomes due the end of the 13th month after you purchase the boat, and they find out about the sale thru the coast guard documentation office or the state department of motor vehicles depending on how its registered (US or State).
The boat US table in the earlier post is not correct with respect to California tax, I think it doesn't count local county and city sales tax, based on where you keep the boat.
Sales tax is one time, but also the county where your boat is berthed applies a yearly property tax based on the price you paid plus the tax (yes, the greating state of CA taxes the tax!) then to top it off, they appear to add "value" to your property that seems to include the boat slip you rent and then tax you for that too!... although this yearly tax is not as painful as the sales tax, the property tax is around 1.5% of the price of the boat. This property tax and sales tax is independent of the sate registration. If your boat is documented by the US Coast Guard, as mine is, in California, you do not need to register the boat per Ca law. I believe this to be a state specific issue, as I recall my coast guard paperwork indicated that a state still has the right to require registration if they chose. So, at least in California at this time you don't register with the Department of Motor Vehicles if you have a US documented boat, and you don't get a sticker or number to put on your boat.
As a side note, I have also read horror stories about people who have moved their boats but the counties still put property tax claims on them even after they move out of their jurisdiction....