When you operate your boat in "Federally regulated waters," it require a registration if you use a motor, regardless of hull length.
If your boat is less than 16' and you are only using an electric trolling motor and you aren't on "federally regulated waters," you don't need to get a registration. (Here in WA).
In other words, If I take my 15' tinny out on Lake Washington or the lower Chehalis River powered by an electic motor, I need a registration.
Same boat and electic motor would not need a registration for smaller lakes or "non-navigable" rivers.
"Non-navigable" rivers are usually waters above the head of tidewater and too small to allow commercial boat traffic.
Is a river guide taking a guest downstream in a driftboat "commercial boat traffic?"
There is a "gray area" here!
Sometimes, on foggy mornings, I will motor upstream from a boat ramp in my drift pram, un-noticed. When I return to the ramp in the full light of mid-afternoon, the motor is off my transom and I'm rowing. These boat ramps are on the lower, tide-affected sections of coastal creeks, and I only use the motor to get upstream as high as I can before rowing back downstream. Much of the skinny upstream water I can squeeze into with electric power would be considered "non-navigable" for a larger boat with a gas motor. Since I use the motor for considerably less than 50% of the trip, I consider the trolling motor not to be my "main means of propulsion" here. The oars are my main means of propulsion. The motor would be nice to have if I popped my shoulder out of its socket, though, and couldn't row. I don't have any plans to ever put a number on this craft. I just need more fog.