There is a tutorial in the Rhino website from the guy that did the Boat that is on their main page. He has a model that you can download to look at his tutorial, and in his model this is what he did.

I revisited it after I posted and the waterline curves, rebuilt at 20 points, actually do deviate a hair from the original curve. Really only at the bow and stern. More points just means harder to work with, but this does give a different angle to adjust the shape in some areas. It may be small enough to still work it this way.

Good luck.