1st point : your curves has many many points ... apply Rebuild for each of them ... I change the scale of the boat ... and apply 0.05 mm in the Rebuild command

this is VERY important to do that before network or other surface tool

2nd point ; it is not possible to cap the hull with only one piece ... it's necessary to split along the Y axe of the boat

i do the same in my exemple yesterday

3rd point ... when i copy the 2 pieces ( cap of the hull ) to extrude ... they need to be flat ... select the pieces and join them

then go to left axe and use the selection handles around the piece and move one to obtain a flat piece ( it says Flat when you reach 0 in z axe

after that extrusion and boolean diff ... the boat is solid ;)

if filleting is not possible ... select the curves of the bastingage .. ctrl c ctrl v ... draw a profile and use Sweep to build a piece ... something smooth or not