Looked up the rules on the Kentucky Fish and Wildlife site. You're right. No mention of a decal for documented vessels, just registered vessels.
https://fw.ky.gov/fishboatguide/Pages/Boating.aspx#2 I''l see if I can't find the tax rules. Obviously it's handled by a different department. Left hand not talking to the right.
By the way, have you seen this article.
https://www.houseboatmagazine.com/20...nt-in-kentucky its an old article but I imagine it is still relevant.
The article makes it pretty clear the Documented vessels are taxed differently than registered vessels. And somehow that works out to the advantage of the documented vessel owners. No wonder they are complaining. If the law is changed they will be taxed at the same rate as registered vessel owners.