Yes, some sort of agora, church, or public square is necessary. I'm not sure that a marina eliminates such a need. I think it expands it.
For optimal sizing of community, see "Dunbar's number". The ubiquitous military unit runs 200-300 people. Companies scaling above this number need much more elaborate process formality etc. etc.
Captured water space is useful for a number of practical reasons with little relationship to the psyche.
I'm not sure I'm clear yet on your "edges are 200% more expensive concept" I'm also thinking that the concept of attachment points corner to corner (vs edge to edge along a significant length) have not been justified or fully baked yet. An important concept here is modularity. I can see a couple dimensions of modular. In the construction phase and modular mobility retained forever. In your concept Ted, it seems a pretty significant engineering feat to attach these platforms, and they probably must stay attached in that specific configuaration. In the hexagon model, the edge connections would also be significant engineering effort and expense. I can't see them being totally hot swappable either, with any edge instantly ready to mate up with any othe edge. But they would be fairly modular from a construction perspective, with each block under construction being built to have one to three connection points pretty easily.
As for displacement, the cheapest structure is probably (as mentioned in the hexagon thread) simply an open bottom with discrete blocks of EPS surrounded by a hull wall that would serve as legs should the structure ever need to be grounded. That's not the most efficient structure from a useful space perspective. But nothing is perfect and cheap is a high priority.
Another question discussed exhaustively on this forum is anchors vs free floating. If you intend to float and anchor, realize that minimum anchor lengths exist. For a standard boat anchor to catch you need to play out a minimum 3 times the depth in anchor line. This also gives you play for tidal variance. Unless you plan to swing around the anchor line, you may need multiple anchors. This can be a potential for fouling other boat in the area or coming to visit your seastead. Depending on your attachment plan between platforms, it may require significant changes to your anchor plan. If you go with higher tech anchor solutions (suction piles for example) the cost goes up and potential environmental impact changes. Actually plan for anchoring to have an environmental impact no matter what you do.
I find th cost extremely unlikely to be financed for anything less revenue dense than a super high end casino hotel.
It'll never happen for the creation of a village. The cost MUST be cheaper. In the realm of tens to hundreds of thousands per family accomodation, not millions. The average person will have to finance their seasteaders move by the sale of a land home and some savings, not debt financing.