It's difficult to say what colour the insides of the bulwarks should be. From examples I've seen, on early boats, (possibly only 19th century boats) everything in the boat was painted white. This can be seen on James Stevens No.14. Moving on, the three post-1920 Norfolk and Suffolks seem to have varnished gunwales, with all boats after them also being varnished rather than painted.
In the case of Watsons, virtually all I've seen photos of have varnished gunwales and natural timber decks.
But unless you have photos that show definitively what it should be either would be okay. I think the earlier the boat was built the more likely it is to be painted white, so you could paint the sailer white and varnish the motor boat.
Andrew