No, but it would be best to do it that way. Any Exposed wood will absorb moisture from the air as well as from the water. The wood then expands and contracts with temperature and humidity. It could also develop rot which could spread. It's best to seal all of the wood, especially the edges. I recently rebuilt the interior flooring and sole of an 18 foot Sea Ray, in which most of the wood had rotted out. I sealed all of the wood. It really didn't cost that much more to do it all, and it will probably never have to be done again. I have also built several wooden dinghies and sealed all of the wood. They are both sitting outside in the pouring rain and I don't have to worry about rot.