In a medieval setting, which methods of fighting a rising sea level exist apart from building barriers? The idea is that a civilization has to fight the flood just long enough to finish their ark.
The civilization has access to iron, coal, stone and wood but is not limited just to those four. Maybe some little magic is possible. The water can come from just one side but it is also be possible that a villiage is located on an island and has to fight the water from all directions. The villiage is always located on the coast and moving into higher altitudes is not possible dua to the speed of the rising sea level.
The reason for the flood is unknown so therfore it cannot be prevented by religion or something like that. The ark is used to save the civilization and to get to a save place when the flood is over or maxed out.