You sketching needs to be more precise. For example, in Sketch 9 you create a tangency condition by snapping the handle to the line. However, I personally don't usually use that "feature" and prefer to see the tangency or "smooth" constraint symbol. That way I can see directly when editing a sketch whether I missed a constraint or not.


I sketch 11 for example, you forgot one such tangency constraint.
also, usually I for curvature continuity (G2), not just for tangency (G1), although for many 3D printed projects, more than tangency is often not needed.
In sketch 13 (1) you use too many fit points. 2 is all that's needed, and you also forgot two tangency conditions. those can create near tangency conditions, which Fusion 360 is very sensitive to.

Last, but not least, at the bottom of the boat, you are lofting into a singularity. NURBS surfaces prefer to be 4-sided.
So one way to battle this is to trim out the offending area and re-loft it with a 4-sided patch.

I projected the edge of the boat to use it as an extrusion profile for capping off the boat so it could be shelled.
Usually I avoid projecting curved edges into sketches, because Fusion 360 has a strong tendency to create horrible curvature. So as a general rule, if you do project curved edges or curves from other sketches, always us the curvature comb to check the curvature for discontinuities.
It's OK in this case:

If all goes well, than the surfaces can be mirrored and stitched and the geometry shells just fine:
