How it floats
A body in water is pushed up by the weight of the water it displaces. A solid block sinks; a hollow concrete box of the same material floats, settling until displaced water equals its weight. Draft is the submerged depth; freeboard is the deck height above water, and it must stay safe under the heaviest load.
Durability
TS EN 206 exposure classes XS (seawater) and XF (freeze-thaw) set the minimum concrete class and reinforcement cover for the hull.