You’re not far off. The Wait Wait… Don’t Tell Me! program that mentioned Paul Allen’s yacht talked about his on-board theater that seats something like 250 people.
A wave motion cannon.
I can’t believe nobody had thought of this one before:
A king sized waterbed
I mean, c’mon! Can we spell “irony”?
Tripler
And what the hell would you do if you sprung a leak? Call “Man Overboard?”
Why a submarine. How else can the kitchen crew tell if all the potatoes are done?
…with state-of-art speakers made right in the building I used to work in.
Robert Wagner?
Roads.
For driving those SUVs on. 
I still think the lake for your other boats takes the cake for sheer (but doable) grandiosity.
Actually, I’m surprised one of the really big cruise ships hasn’t already installed a ‘subway’. If you only had one small ‘subway car’ and automatic operation with call buttons at each ‘station’, it would be more like a horizontal elevator than metropolitan transport… and there are plenty of ships with elevators.
If you designed it right, the subway cars could operate in the elevator shafts as well.
Are there special requirements for elevators in marine usage? What happens if you’re on one and the ship starts to sink? (I can’t believe I never thought of this when I was in the ferry between Helsinki and Stockholm… that ship had several elevators.)
How about…
A Great Glass Elevator? And a Chocolate Factory to put it in, of course.
A gyroscopically stabilized deck to avoid seasickness and allow pastimes such as golf, pool, etc.
On that deck there should be a large swimming pool, and an even larger wave pool - generating waves of exactly the size and shape that your passengers want.
This touchs on a couple of mentions here, but the original glurge that started this whole thing claims that Paul Allen’s toyboat has a crew of 60 that are allegedly all or part ex Navy Seals. There is a large swimming pool (saw the photos) and a flight deck, in both the front and the back. The back has a hangar for the helicopters. And there is a submarine that allegedly can sleep 8 for several days, IIRC. For checking the potatoes and whatnot.
Speaking of which, we’ll be needing a pitch-n-putt. Nothing fancy, just one or two par-threes.
Pitch-n-putt? Please; that’s so boring. Duane Hagadone’s yacht Lady Lola has an 18-hole golf course. (There’s a place to tee off on deck, and then the crew sets up 18 bouys to serve as targets. Closest to the flag wins that hole. The balls float and are retrieved by the crew.)
And apparently the barstools on Aristotle Onassis’s yacht were upholstered in whale foreskin.
Torpedos for self defence? Damn the torpedos.
Actually, you might take the logical step from the above system would be to add huge wheels or treads to the yacht, so not even the land could impede it’s passage.
A horse stable.
Indoor shooting range.
One chamber filled completely with gold bricks, then sealed off completely from human access, for absolutely no reason.
A foundry.
A Dolphin tank. (You can teach Flipper to whistle “Folsom Prison Blues”)
A forensic laboratory.
A Starbucks franchise.
A Triumphal Arch.
Mausoleums. (Fore and Aft)
Private gallery of wax figures, by Madame Tussauds.
Dissection Theatre.
A large gravitational field.
Throatwarbler Mangrove?
Ha ha. My votes here.
Imagine getting all that other stuff and finding out that you forgot to pack the batteries.
Whaddya mean, for absolutely no reason? That’s ballast.
What I’d want would be a wet-deck, so I never have to worry about high seas or rain when I want to use a launch to ashore in smaller ports without proper dock facilities for my moving island.
There has to be a joke in here, but I… can’t find it.
Yes! Yes! YES!!! When I’m rich enough to have a yacht, that’s what it’ll be called! Although I’m sure someone else got there first.
I don’t think I’ll bother with the 30ft prosthetic nose on the bow, however.