The dock was made of concrete but filled with foam. This type of construction makes it almost impossible to sink.
Perhaps this type of construction, combined with Ellmer’s ideas of semi-submersible living space, would work well for a seastead. Expensive, but not mind-bogglingly expensive. With the bulk of the mass not susceptible to wave action the platform should be pretty stable, and done right it should not sink even if filled with water. Apparently the foam-filled type of construction was used by the U.S. freshwater navy in Vietnam, and the boats would not sink even when filled with holes. This sort of construction would make me much more comfortable about living on a semi-submersible. For one thing, with a traditional submarine, if it filled with water residents would have to swim to safety immediately or be quickly dragged to a depth too deep to escape. If this sort of semi-submersible filled with water, one might have to swim from a depth of 30 feet, and have a platform to rest on after the escape. That makes a huge difference in recovery from a disaster.