Are trying to ask “How can you defend a homesteaded space on the sea?”
I think the problem with your question is that you are placing too many conditions upon what constitutes homesteading.
If you are just asking “How can you homestead something like the sea?” then the answer is simple: you homestead the sea by simply declaring a certain three-dimensional area is yours. In fact, whether we recognize it or not, homesteading of the sea or “seasteading” is done all of the time whenever a vessel sails. By default, the vessel is claiming sole authority over the control of a certain physical space that just happens to be on water. Take note:
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That space is the area occupied by the vessel.
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That claim is constantly changing as the vessel sails.
In essence, homesteading the sea is not much of an issue. It just adds dimensions to a property claim.