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Billionaires don't exactly have a great track record in submarines. A crazy megayacht, or even a regular yacht, might serve you better!

Is there something inbetween ? A yacht with optional submerging feature ? May be just 100 feet ?

All yachts can be optionally submerged. Once. Surfacing afterwards is the tricky bit.

"Anything is amphibious if you can get it back out of the water." (Schlock Mercenary Maxim #32)