Interpreted from the Washington Boater's Guide-

Boats 16' and above need a license, motorized or not.
I just bought a 14' boat last year with no motor and had to license it because it had a max horsepower capacity of 35. If your boat is smaller than 16' with a maximum hp capacity that is less than 10hp you do not have to license it. A 10hp motor is the cut-off on any size boat.

Check the Coast Gaurd tag on the transom.

Hope this helps! smile
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Carl C.