Hmm. I think private interests will indeed develop oceanic resources in the coming century. They have already started in fact. That's because the ocean is fairly cheap to access and retrieve valuable resources from, compared with space.
Space is the real problem, for anybody whose perspective is anti-government. How do you fund large human colonies on the Moon or Mars without any government backing? What will you do on the Moon or Mars to pay for it?
Thanks to robots, there is no profitable activity that can be done in space which requires a human presence. Asteroid mining can be done by robots. Satellites are unmanned. There's a market for sub-orbital joyrides for the wealthy, but that doesn't accomplish the goal of putting millions of humans on other planets.
Yes, SpaceX and other private spaceflight companies exist. But they have not sent humans into space yet, and when they do, it will be because NASA (a government agency) pays them to do it. The existence of private entities that supply space access as a service to NASA isn't new: The Apollo program and everything after it was facilitated by the likes of Lockheed Martin and Boeing, for example.
The only thing new about SpaceX is that they do not receive cost plus contracts, so they have to deliver the service they promise to at the price they quoted from the start. This is a good development, but it does not solve the fundamental problem.
The problem is that the only reasons humans have ever been sent into space is because the government forced it to happen. There never was and still isn't a way to profit from populating space with human beings.
The ISS for example was created by government dictate, simply for there to be a destination in space to send humans to. Without that government dictate, the ISS would not exist because there's nothing to do in orbit that generates a profit, which also requires a human presence.
Even if gold bricks were stacked up eyeball deep on the Moon, it would cost more to retrieve them than they are worth. Mining on Mars or the Moon only makes sense if it's for the benefit of an already existing colony of humans on the Moon or Mars. There is no way to sell those metals to Earth at a profit.
But how do the colonies get built, if not by arbitrary act of government? I think a more balanced, compromised view works better here. Government should only do what is necessary, but cannot be done at a profit. This is for example why the US postal service still exists, to deliver mail to remote locations that UPS and FedEx won't, because it's not profitable.
I'd argue that space colonization is in that same category. It needs to happen but there's no way to make it self-supporting, financially, for the reasons outlined above. Corporations therefore won't do it, not without the government using taxpayer money to force it to occur, subcontracting parts of the project to said corporations.
I think the dreams of commercial rocketry from the 1950s science fiction stories are clearly dead, but I think there are enough people with an instinct to explore the frontier to make lunar and martian colonies inevitable in the long run, whether because overpopulation becomes a real concern or because governments are too oppressive. The logistics are a nightmare though.
The ocean is indeed an unexplored frontier as well, and there are several interesting seasteading proposals. Some are explicitly aimed at using international waters to escape governmental overreach. Logistics aren't much of an issue, comparatively. The technology has been around for decades.
Alright, you've proposed two possible pressures that may result in space colonization here.
The first one is that a lot of people want to colonize space. However I don't think that will get them anywhere. It's incredibly expensive to colonize space. A bunch of space enthusiasts can't overcome that barrier with passion alone.
The second one is overpopulation. The problem here is that space colonization is not and never was a workable solution to overpopulation for the simple reason that even if every country on Earth was launching rockets at the rate of one per minute, it still would not move humans off the Earth faster than new ones are born.
I agree the technology exists, but expense is the barrier. It is not trivial, either. Ignoring the high cost of something doesn't make it cheaper, and even if one is able to raise the money from donors, if the project doesn't generate more income than it costs to maintain, it will not be permanent.
This is why, as I have written in the past, nearly every underwater habitat ever built was scrapped to recoup some of the cost by selling off the metal it was made out of. None were simply left in the water.
Out of the three in operation today, one is operated with university funding (Aquarius, formerly government funded) the other two are the Jules Undersea Lodge which pays for itself by hosting tourists, and Marinelab (in the same lagoon as Jules, owned by the same guy) which is supported by revenue from the operation of the Jules as a hotel.
I do not mean to be a downer. But this is the difference between dreamers and executors. The world has plenty of dreamers. It has very few people who can actually make those dreams happen, and fewer still who can make them happen in a lasting way.
This is because at the end of the day, stuff like underwater bases, space stations and so on are very expensive. It has to be paid for somehow. If it's not going to be paid for with tax dollars, then it needs to generate profits somehow to pay for itself.
I don't think overpopulation is nearly the threat many present it to be, and even if it were, I don't think space colonization would be a cure for it, just an incentive for some to pursue it.
And don't get me wrong, I don't mean to trivialize the cost of oceanic exploration or colonization either. However, since government has for the most part been behind it thus far, we don't really know what the real prices or potential innovations are in the industry waiting to be unlocked.
For the moment, I don't think sea floor habitation is likely, but The Seasteading Institute offers some interesting ideas, and this interview explores some of the ways it could generate a productive profit.