I like the idea of an undersea utopia, but I don't think I'd enjoy the reality of funding and developing it enough to want to spend my fortune on it. It sounds like a lot of work and more than a full time job, you can't just throw $100M at your cause and expect it to be done in a way that meets your vision, you need to be there to oversee and guide it and make sure your money is being spent wisely. (a good example is Disney's EPCOT -- Disney's vision was to make it a real living city, after his death it was turned into just another theme park)
A home in the Mediterranean and a yacht sound much more inviting, not to mention less risky than living in an undersea habitat that I just built.
Edit since I’m getting downvoted: yes the ultra rich can and do live off debt with their stock as collateral, but that gets you to the “merely” living a luxury lifestyle point. It’s not a viable mechanism for funding SpaceX, Blue Origin, Virgin Galactic, etc, which is the kind of “interesting stuff” this thread was talking about.
They can deduct dept/interest payments from taxes and never actually have to sell a lot of their stock. That way their wealth can accumulate and they don‘t really generate income. Almost like the tax code is skewed in their favor.
The interest rates aren't particularly important in this concept. Just that you have enough collateral so that you can life off the loan while the collateral appreciates in value, so that you're not forced to sell it.
Maybe an accountant can chime in.