Communities are strongest when united by shared ideals, and it seems quite likely that early seasteads will exist largely for ideological reasons. This gives an additional reason why they will be communal in nature. Also, being isolated will increase interdependence among seasteaders. Note that communal does not necessarily mean a communist society where all property is owned jointly. It simply means that the line between what portion of property is public and private will likely be a bit farther than in normal society. People's lives and property will be more intertwined.