That's the annual "excise tax." The use tax is only due in lieu of sales tax if you bring the boat into Maine within your first year of ownership. Here is some material from the Maine website:

ANNUAL EXCISE TAX

Before a watercraft can be registered, the owner must show proof that the annual excise tax has been paid on the watercraft. Maine residents pay the tax to the town in which they reside. Nonresidents or corporations pay the tax in the Maine town where the boat is principally moored, docked or located. All watercraft, including documented vessels, located in Maine are subject to an annual excise tax except the following:

A lifeboat or raft carried by another vessel.
A demonstrator of stock in trade (dealer's stock).
A commercial vessel with no established base of operation in Maine.
A watercraft not in Maine more than 75 days during the year. This refers to the number of days physically present in the State, not the number of days in use in Maine (Non-commercial vessels only).
A watercraft 20 feet or less in length and does not use any type of motor power.
A watercraft exempt from property tax.
Any watercraft not subject to registration but required to pay the excise tax must display an excise tax sticker on the watercraft to show that the excise tax has been paid.

SALES TAX

The first time any owner of a watercraft registers the watercraft, the owner must (1) show proof that a 6% sales tax or use tax has been paid, or (2) pay the sales/use tax due to the Registration Agent at the time the registration is issued. Sales/use tax paid to another jurisdiction will be credited toward the Maine sales/use tax due. A watercraft purchased, registered and used by the present owner outside of Maine for at least a year is not subject to sales/use tax as long as the owner was a resident of the other state at the time of purchase.