Heights come from the source map's colour bands, so every cell is a 0.5 m band, not a measurement. Height is exaggerated about 3,000× so a 4 m range is visible across 120 km.
The water is still water. It ignores embankment crest heights, pumps, rain and flow. The land has also kept sinking since 2010, so today's ground is likely a little lower than shown.