Well, you could use geothermal energy or make use of the temperature gradient between the top of the ocean and the 100m down. Its true the that pressure differ

Well, you could use geothermal energy or make use of the temperature gradient between the top of the ocean and the 100m down. Its true the that pressure difference between vacuum and sea level is much greater than between sea level and deep underwater. However you have the advantage of not having to lift everything up to orbit. You you can build a really heavy steel tank using relatively 'simple' construction techniques, then just tow it out to position, before sinking it and anchoring it to the seabed. The more I think about it, the more it sounds like like a self-sustaining undersea colony would be a affordable and useful way both ensure species Survivability and act as a prototype for a space colony.