Have a look in the main Mayhem sit in "Your Models" and scroll down to "Malcolm ..... Gytha".
The odd bits are probably items left in from when it was sold as a motorisable kit. Mine had a tiller arm that got used. The only extra waterproofing needed was to fit a high coaming for the superstructure to sit over.
The deck edge was properly and fully sealed with styrene cement.
No hole in the after deck (why, for goodness' sake?) - thats where the batteries live, two each side of the prop shaft, and the rudder linkage is carried over the deck. Painting it red helps it to vanish. The tiller arm lives under the grating which is retained by a bit of blue-tack. This means that the working bits are accessible for servicing.