Using established technology from offshore industry it's supposedly not as crazy nor costly as one might first imagine.
There was a nice and fairly accessible talk[2] given at Perimeter Institute, which gave some background and went into this and the FCC.
[1]: https://arl.physics.tamu.edu/research/collider-in-the-sea/
However 100m depth? For a structure _that_ size, which I think might contain high vacuum, let alone systems that might need humans to service when they break? What is the feasibility of that problem?
Also mentioned is that the LHC cryostats, some nice illustrations here[1], are very nearly neutrally buoyant. Given these would be built similarly, they wouldn't need significant infrastructure to keep them in place.
So no humans would need to go down there, if something breaks they could just replace the broken segment(s) and fix the broken one(s) on the surface.
[1]: https://cas.web.cern.ch/sites/default/files/lectures/bilbao-...
This frankly is not possible. Colliders need constant maintenance. The "Collider in the Sea" is not a real proposal.
The first unfounded assumption is that, regardless of what the LHC found, there was always going to be a need for a larger collider of the same design. This is clearly false - the design of the FCC (for better and worse) is specifically geared to confirming or probing various discrepancies that the LHC found, and to finding things that the LHC was hoped to find that didn't pan out. If the LHC had found SUSY partners or WIMPs, or if it found no conflict with theory whatsoever, the next accelerator might have had a radically different design.
Secondly, you're assuming that it's possible to build a collider that has (significantly) higher power than that with foreseeable technologies. The reality is that we are very close to the limits of what seems achievable in terms of particle accelerators, and per the people at CERN, even this 6x increase in collision energy will require ~40 years of research to be achieved. And the collider we need to probe regimes where we really have good scientific reasons to believe will reveal new physics (i.e. the regimes where we know the standard model breaks down) is not 15x or even 100x higher: even a collider buried around the entire equator wouldn't get anywhere close, we'd need a collider the size of the solar system to actually probe these regimes. So going 6x or 10x or 50x is irrelevant, unless you're looking for some very specific technicalities, like they are at the FCC.