Schmism - I've been thinking about your parts and wonder if they are really plastic...or could they possibly be carbon fiber. I think a lot of such parts are often made from CF. CF has a more "grainy" look than plastic...which is usually high gloss.

The crane and spreader vary from kit to kit what they are made out of.

The Xacto will gut the hatch opening...but probably take several strokes. Do it neat as you will probably have to use the piece you cut out. Dremals have a tendency to run away with things sometimes.

The waterline??? Up to you. Since you are doing the "blue transition" thing, I don't know that I would worry about the exact waterline. Keep in mind sailboats seldom sit around on an even keel anyway and are normally rolling over this way or that. So the "waterline" appears lots of places. A boat the size of yours will probably sit 1/2 to 1" deep in the water and slightly down by the stern is my guess. I wouldn't knock myself out finding the "exact" line.......which you won't really know till the boat is built and loaded w/radio gear.

I suggest you paint it NOW.