SouthForkSam says...

Should I worry that Black Walnut is only 1010 on the Janka hardness
scale where White Oak is 1300 and something?

Maybe I should just use Purple Heart or Padauk (both higher on the
hardness scale than white oak)...... but I really am leaning towards
the color of the walnut. I can already see the dark brown, brightworked
gunwale contrasting against an ivory paint on the outer hull.

I'm not a boat builder, but it doesn't seem like a big deal considering
the application and considering you are going to put some tough finishes
on it. You could use Jatoba as well. Very hard and durable and it has
a nice color.