They should just call it what it is ... a tax, and then collect it like any other tax. Seriously, what "service" are you really getting by paying to have the vessel registered that isn't related to paying taxes somewhere ? It's like a tax so you don't have to pay some other tax.
The U.S. get out of other taxes tax.
Your boat won't sink without one, so why not call it what it is and be done with it.