Looks like they made a new law

Looks like it includes trailer boats also.
Vessels that are registered in another state and are operated on Indiana waters for 60 consecutive days or less.
Beginning in 2007, out-of-state vessels also must pay the Indiana Boat Excise Tax in order to be exempt
Indiana law requires that vessels that are operated, used, docked, or stored in Indiana pay the Boat Excise Tax.
Vessels subject to excise tax include:
Motorboats registered in Indiana
Sailboats
The amount of boat excise tax paid is:
Based on the value of the vessel when new and is reduced 10% each year up to a 50% reduction of the original amount.
$12.00 for vessels that are stored in Indiana for 60 consecutive days or more in a boating year but not used or docked in Indiana waters.
When you receive your certificate of registration, you also receive two excise tax decals.
Decals include the date of expiration. Only the current decal may be displayed.
The boat excise tax decal is valid for one year. Owners of registered vessels will be sent a renewal notice to their residence automatically.
If you lose or destroy your decals, you must apply to the BMV for duplicates and submit a processing fee.