The World isn't a regular cruise ship. It consists of 165 apartments mostly owned by residents. A few years back the apartment price was quoted as between $2m and $15m.
I keep wondering how many guests are usually on the ship, given I assume most people who'd spend millions on a flat on a cruise ship probably have residences elsewhere too. Keep imagining it as a near ghost ship (which is not true of course - it has a crew of 280) and a monument to excess.
In a surprise to no-one it's registered in Bahamas (no income tax, corporate tax, capital gains tax, or wealth tax). I suspect most people with flats there won't spend long enough there each year for it count for tax-residency (some, maybe), but it's certainly a benefit that if you were to the tax is non-existent.