I made a Sugar Island out of Douglas fir a few years ago, and I tried something a little different for the shaft.
It started with my normal 1.25" square, followed by lines drawn on with a spar gauge, followed by the rendering of an eight-sided shaft using a small block plane and spokeshave. But that's where I left it. I knocked the edges off and rounded them a little, but it was clearly an eight-sided shaft-- and curiously, it feels very nice in the hand-- lots of feedback; it's tactile, alive.
The octagonal shaft is full dimension for 16" above the throat, and then tapers to roundish toward the grip. I wish I had a photo, but a dear friend fell in love with the paddle a couple of years ago, and well, you know how that goes.
You might be surprised how compelling this shape feels, and if you don't like it, just finish the shaft to round or oval.