Executive Summary: Deploying a single-family seastead with temporary tension-legs via helical screw anchors presents a novel intersection of maritime law. Because the technology bridges the gap between a vessel (which anchors) and an installation (which is fixed), the legality will largely depend on how you frame the technology to local harbor masters. The environmental benefits of this system are your greatest asset.
In international and local maritime law, the duration of the stay and the intent of the deployment generally dictate the classification. If your seastead is capable of navigation (even if towed), moves after a few days, and removes its anchoring apparatus entirely, it legally functions as a vessel dropping anchor, rather than a permanent installation.
However, the method—screwing into the seabed—may visually and mechanically mimic permanent moorings (like oil platforms or eco-moorings). If authorities see you using mechanical torque to embed anchors, their reflex may be to apply "permanent installation" regulations. Proceeding with transparency and clear terminology is key: do not call it a "platform" or an "installation." Call it a "vessel utilizing advanced temporary eco-anchors."
Have there been many of these used non-permanently?
Currently, the non-permanent use of helical screw anchors by individual vessels is rare. Most vessels lack the specific hydraulic or mechanical equipment required to install and remove them quickly. Helical anchors are predominantly used for permanent "eco-moorings"—where they are installed once, and visiting boats simply tie off to the buoys attached to them.
Have people run into legal trouble?
Legal trouble regarding anchoring in the Caribbean almost entirely revolves around seabed damage. Yachts and ships face heavy fines for dropping traditional anchors and dragging heavy chains across coral reefs or protected seagrass beds (e.g., Posidonia or Turtle grass). Because non-permanent helical anchoring is virtually non-existent for transient vessels, there isn't a long history of legal trouble specifically for this method. Your hurdle will not be past legal troubles, but rather bureaucratic confusion over a new technology.
Initially, yes. In the vast majority of Caribbean countries, if you clear customs and immigration and pay your standard cruising/anchoring fees, the authorities rarely inquire about the specific geometry of your anchor, provided you are anchoring in designated sandy areas.
As long as you are registered as a vessel and operate within the standard time limits of a cruising permit, standard anchoring rules technically apply to you. However, if harbor masters observe the complex deployment of tension legs, they may ask questions. Your defense is simple: "This is my vessel's anchoring system; we remove it when we leave."
Eventually, highly likely. Bureaucracies respond to novelty by creating new categories (which often carry higher fees). If seasteading becomes a recognized niche, or if multiple vessels start utilizing tension-leg setups, island nations will likely create a "Temporary Moorings/Stationary Vessel" permit.
There is also a risk that if this system is deployed near marine parks (like those in Bonaire, the Bahamas, or the USVI), authorities might require a special environmental impact assessment to prove that screwing into the sand does not disrupt micro-ecosystems, even if it is safer than dragging a chain.
You mentioned that you will likely need to "go to different places, explain it, and see what they say." This is exactly right. Here is the framework for that conversation:
Your tension-leg seastead concept solves a major problem in marine stability and environmental anchoring damage. Legally, because it is temporary and removable in minutes, it completely fits the spirit of "dropping anchor" as a vessel. By educating local authorities on the environmental superiority of a vertical screw versus a dragging horizontal chain, you can turn potential regulatory suspicion into enthusiastic governmental support.
Disclaimer: The information provided in this document is for educational and strategic planning purposes only and does not constitute formal legal advice. Maritime law varies significantly by jurisdiction and is subject to the discretion of local maritime authorities and port captains. It is highly recommended to consult with a certified maritime attorney regarding vessel registration and classification before deployment.