Mega-yachts.
Here is a yacht buying/selling website with a filter for mega-yachts: https://www.yachtworld.com/boats-for-sale/type-power/class-power-mega/
The most expensive ones listed there are in the lower hundreds of millions (US$). I didn't check if these were new or used. The smallest ones I would start to call mega-yachts were mid-to-upper hundred of thousands.
There's also this calculator site for the purchase annual operating expenses of superyachts: https://www.luxyachts.com/yacht-cost-calculator
Buying an island:
https://www.privateislandsonline.com/ lists many islands. They're between about $100,000 and $75,000,000 (some lower-end ones are undeveloped and just trees), most seem to sit in the millions range. There are also renting options but prices aren't public.
This advertisement blog post shows similar prices: https://www.jamesedition.com/stories/real-estate/how-much-does-a-private-island-cost/