Everything has to be salt-proof. Corrosion even in coastal homes is huge. On water everything starts to rust within days. Plus exponential growth in size - larger boat means bigger masts, bigger sails, bigger winches to move it, more power to winches, bigger generators, larger fuel tanks.

You should compare it with airplanes which would make it sound like bargain...

I guess it’s because of much lower use of off-the-shelf materials.

Partly that’s unavoidable. The hull has a unique shape, and that means you can’t just put standard kitchens, bathtubs, etc. in if you want your design to shout ‘luxury’.

For yachts in this category, I guess even the door handles, flooring, etc. are custom designed.

This ship also wil have huge carbon masts that are unique in the world.

That adds up. I guess Apple’s spaceship office similarly is quite a bit more expensive per m² than your regular office building.