Pefect just make the same that on my image above!

Create a solid box

Put the boat just a little inside
Boolean Union : you have a solid!

Draw a line any where you want but just under the box
Boolean Diff = you have your Hull solid

And you can now make a Boolean diff between your new hull
and the same hull some scaled
as shown on previous post



And after you can add 1 mm or more if you want! ;)
Move the face for make the little Shell
But all that because your curves were not perfect from the beginning...
A little shell