As per SOLAS chapter II-1 , regulation 4,
Floodable length is the amount the ship can be flooded without the risk of the ship sinking. The floodable length is the maximum length of the ship that can be flooded without submerging the margin line, also known as the deck line. The margin line is a line just below the top of the bulkhead deck, or the highest deck which has watertight bulkheads forming watertight compartments. If the margin line is submerged, the ship is serious danger of sinking from flooding.
This can result in a dangerous situation called down flooding where watertight compartments are filled from the top causing a loss of buoyancy and the ship to sink.
Each ship has a unique floodable length and most are designed such that the spacing of bulkheads will allow two adjacent watertight compartments to be flooded without submerging the margin line and endangering the ship.