Marking in a fair curve to the floorboards requires a long, flexible strip of some kind. I used a length of wooden edging.
Here is the batten held in place by spring clamps against nails pushed between the floorboards.
When the curve was marked to my satisfaction I removed the boards one by one to do the final trimming with the jack plane, and then replaced them to check the final fit.
Here are the trimmed boards, all in place.
Now I can stand in and move around the boat with ease. Hooray!!!