Note the British court determined Sealand was not under British jurisdiction because it was located in international waters. It did NOT hold Sealand was a sovereign nation. Sealand has gotten by because it hasn’t been worth anyone’s time to push the issue. As pointed out above, not being part of a sovereign nation also means you don’t get the associated benefits.
If the issue was pushed in a court, I believe they would find Sealand or your hypothetical “installation” to not be a sovereignty, but rather personal property or a fixture to international waters and land. Therefore, international admiralty law would apply. I find it interesting evidently Sealand can’t keep the peace in a population of a dozen or so, any better than any country can.
If there was ever any really serious illegal activity going at Sealand, I am sure the British government would not hesitate to just take it. In fact, it might not even be too difficult to get a court decision declaring the “residents” of Sealand to be pirates, or at the most, squatters. Who is going to complain? The United Nations?
A very interesting related issue is that the recent thawing of the Arctic ice cap, if it continues, will finally make a Northwest Passage above North America and Asia a viable sea route, which will not only shave huge margins off long distance shipping costs, but may also finally make access to the far northern oil, gold, uranium and other resources practical. The U.S., Russia, Canada and a number of other countries are already starting to duke it out, using legal arguments resting on centuries old explorations, submarine expeditions under the polar ice cap, et cetera. Should be fun to watch.