This is from the DOL web site.

What size boats with a motor are required to be registered?

All vessels with propulsion machinery are required to be titled/registered including sailboats 16 feet and over without any propulsion machinery. For information on other exemptions, please contact your local county auditor's office or closest vehicle licensing office.

Here's the link
http://www.dol.wa.gov/vs/vs-faq.htm#size

I wish it wasn't the case, but there gonna get us any way they can.