They didn’t build the boat out of bank notes. The $500M is in the pocket of the thousands of people involved in the supply chain for the shipyard. Much better use society-wise than keeping that value parked in financial instruments.

While I do agree that having money put to use is preferable to not, the argument that any spending is therefore good seems somewhat absurdist. We wouldn’t apply that logic to government spending, for example - the government gold-plating Air Force One produces different effects than the government building water sanitation, even though they both result in economic activity. Accordingly, some spending produces more social good than others. The argument that critics of spending like this isn’t that it would be better for the money to sit in financial instruments, it’s that the money would be better off partially captured via taxes and more directly allocated to social good.

Others have already commented in a similar manner but I think this is worth driving home since the argument you provide is so typical when it comes to these discussions.

The money spent on the yacht has improved society.

The evidence of such is that the public officials in Rotterdam are willing to temporarily move a bridge so that construction of the yacht can continue within their municipality. They see it as a greater benefit to their society to encourage the construction of such yachts. Oceanco employs 320 people whose salaries are likely taxed at a fairly high rate. They have two shipyards in Rotterdam on which they probably pay significant property tax among other fees. They are a manufacturing business and yachts consist of a huge variety of different materials and finished goods so they will be deeply integrated with the supply chain which means there is a large outflow of cash from them to other businesses. The production of luxury goods has a significant impact on an economy, by their nature they demand the highest quality parts and workmanship. Attracting those materials and skills to a city has many knock-on effects that are beneficial. If you removed the demand for or the option to buy super yachts, Rotterdam and other cities would suffer a loss of jobs, skills, tax income, and probably several other businesses.

I’m sure the boat builders are very happy to take the money. It’s not like the money just disappears.

The only resources being used up here are one boat’s worth of metal/plastic/energy/whatever. Probably about as much as a couple of hundred cars.

Money is fake, it's an abstraction made of cheap throw away goods (what money is printed on) used as a representation for real resources. I personally wouldn't count it as relevant but it does buy access to what is relevant.

I would add to your list, some of the finite resources I can think of that's wasted here is labor per human per year. The labor of those boat builders. It's an opportunity cost type of waste.

That and many hours of skilled work that could be applied elsewhere.

You could add “luxury yacht builder” to the very, very long list of jobs that exist only to provide entertainment to other people.

Indeed, but those people could make luxury yacht for 100 people, dividing uselessness by 100.

The world is probably full of things that shouldn't exist by that standard. Do we "need" all the big cathedrals, pyramids, temples, expensive to make works of art to name a few?

Well, it's his money.