Has to be a less than optimal stencil value used for the mask.
Are you doing any math to the output when isolating the stencil?
Also, to do double shell you normally need 2 shells. I only see one…
So thats confusing.
The inside of the boat mesh is the one that goes in the stancil.
The outside is left alone or it will look as it kind of does floating in air rather than in water…