In my notion, the hexagon has an internal deck slightly above water level. It has walls that extend above and below this. Height is somewhat irrelevant but for the moment, consider an average height of 20' overall, with half above deck and half below. The underwater portion has an open bottom. The deck is supported by sealed blocks of Expanded Polystyrene (EPS) foam also known as styrofoam, providing buoyancy. The mid-point deck provides structurally integrity, and the top is covered by a "weather deck" that supports superstructure or living/working space.
Should the water tight integrity of the shell between the lower deck and weather deck be compromised, there is no worry because the EPS positively displaces, unlike a hull enclosing empty space.
The multiple hexagon configuration is merely a means of attracting population with single family residences. By using hexagons in a modular fashion, individual units become affordable and cost-effective at the individual family level and become an asset with ownership equity. Clustering them provides for some off-loading of common functional needs like freshwater storage, waste treatment etc. to dedicated hexes (either publicly funded via "taxes" or by service subscription to a business that owns the hex. Variable configuration within the basic size and shape restriction allows for better fit to purpose as well as aesthetic variance. For example, your hexagon could be taller or shorter than mine (above water or below) and still fit into an overall village design.
- By using a substantial straight edge, a robust mechanical connection can be achieved (better than corner->corner connections or round shapes).
- The hexagon shape naturally lends itself to multiple units connecting to each other in repeatable patterns while also achieving considerable trapped or en-trained water space.
- Hexagons packed densely have good structural strength against wind/wave loading or tow operations (being pushed by a tug).
- The semi-irregular outside edge of a shape formed by multiple hexagons lends itself to greater linear edge than squares or rectangles of the same area, and better interruption of wave patterns than longer, straight edges.
- Nature has few straight lines; biomimetic design of artificial ecosystems benefits from transition zones and irregular edges. Shellfish aquaculture and similar activities will benefit for the point above as well.