It's not hard to make an unsinkable concrete canoe. I did in college. We used a cement, water, polymer & glass bead aggrate mix that had a unit wieght less than water, so it floated without any added foam floatation necessary. It was also 12,000 psi, about 3 times as strong as the concrete used for house foundations, and was also relatively flexible.
The glass beads and the polymer made the big difference. The beads are the same used in the road line striping paint to provide the reflective quality.
Last I knew, several school, especially University of Alalbama, Huntsville, were using carbon fiber reinforcing and space age polymer "concete" at about 3/8" thick to make canoes that were lighter than any commercially available canoes.
So the dream was to build a sailboat with the same characteristics....and having read about "rogue waves" (on Free Republic of course!), plus the danger of ramming something, or being rammed, I thought the ferro cement concept would be the ultimate in safety. Not surprised that the aggregates & polymers have advanced in the years since. FRegards,
john